PARKLAND (AP) — A former student opened fire at a Florida high school Wednesday, killing at least 17 people and sending scores of students fleeing into the streets in the nation's deadliest school shooting since a gunman attacked an elementary school in Newtown, Connecticut.

The shooter, who was equipped with a gas mask and smoke grenades, set off a fire alarm to draw students out of their classrooms shortly before the school day ended, officials said. 

Authorities offered no immediate details on the 19-year-old suspect or any possible motive, except to say that he had been expelled for disciplinary reasons. 

Frantic parents rushed to Marjory Stoneman Douglas High School to find SWAT team members and ambulances surrounding the campus. Live footage showed emergency workers who appeared to be treating the wounded on sidewalks. 

"It is a horrific situation," said Robert Runcie, superintendent of the school district in Parkland, about an hour's drive north of Miami. "It is a horrible day for us." 

The suspect was taken into custody without a fight about an hour after he left the scene. He had at least one rifle and multiple magazines, authorities said. 

"It's catastrophic. There really are no words," Broward County Sheriff Scott Israel said on Twitter. 

The attacker used the fire alarm "so the kids would come pouring out of the classrooms into the hall," Sen. Bill Nelson said in an interview on CNN. 

"And there the carnage began," said Nelson, who said he was briefed by the FBI. 

The Florida Democrat said he did not know if the gunman used the smoke grenades, but he assumed that's why he had a gas mask on. 

Most of the fatalities were inside the building, though some victims were found fatally shot outside, the sheriff said. 

The gunman was identified as Nikolas Cruz by a U.S. official briefed on the investigation. The official spoke on condition of anonymity because he was not authorized to discuss the information publicly. 

Freshman Max Charles said he heard five gunshots. 

"We were in the corner, away from the windows," he said. "The teacher locked the door and turned off the light. I thought maybe I could die or something." 

As he was leaving the building, he saw four dead students and one dead teacher. He said he was relieved when I finally found his mother. 

"I was happy that I was alive," Max said. "She was crying when she saw me." 

In a cul-de-sac near the school, Michael Nembhard was sitting in his garage when he saw a young man in a burgundy shirt walking down the street. In an instant, a police cruiser pulled up, and officers jumped out with guns drawn. 

"All I heard was 'Get on the ground! Get on the ground!'" Nembhard said. He said he could not see the suspect's face, but that the man got on the ground without incident. 

The day started normally at the school, which had a morning fire drill. Students were in class around 2:30 p.m. when another alarm sounded. 

Noah Parness, a 17-year-old junior, said he and the other students calmly went outside to their fire-drill areas when he suddenly heard popping sounds. 

"We saw a bunch of teachers running down the stairway, and then everybody shifted and broke into a sprint," Parness said. "I hopped a fence." 

Beth Feingold said her daughter, Brittani, sent a text at 2:32 p.m. that said, "We're on code red. I'm fine," but sent another text shortly afterward saying, "Mom, I'm so scared." 

Brittani later was able to escape the school, which is one of the largest in the state, with about 3,000 students. 

Inside, students heard loud bangs as the shooter fired. Many of them hid under desks or in closets and barricaded doors. 

Television footage showed students leaving in a single-file line with their hands over their heads as officers urged them to evacuate quickly. 

Caesar Figueroa said when he got to the school to check on his 16-year-old daughter, he saw police officers drawing machine guns as they approached the campus. 

"My wife called me that there was an active shooter, and the school was on lockdown. I got on the road and saw helicopters. ... It was crazy and my daughter wasn't answering her phone." She finally texted him that she was inside a closet with friends. 

Len Murray's 17-year-old son, a junior at the school, sent his parents a chilling text: "Mom and Dad, there have been shots fired on campus at school. There are police sirens outside. I'm in the auditorium and the doors are locked." 

A few minutes later, he texted again: "I'm fine." 

Murray said he raced to the school only to be stopped by authorities under a highway overpass within view of the school buildings. He said he told his son to save his battery and stop texting. The boy's mother told him to turn off his ringer. 

Murray said he's had just one thought running through his mind since he got his son's text: "All I keep thinking about is when I dropped him off this morning. I usually say, 'I love you,' and I didn't this morning. He's 17, he's at that age, and I didn't say it this morning, and I'm just kicking myself right now over and over and over. Say it early and often, I'm telling you." 

The school was to be closed for the rest of the week. 

The Newtown attack shocked even a country numbed by the regularity of school shootings. The Dec. 14, 2012, assault at Sandy Hook Elementary School killed 26 people — 20 first-graders and six staff members. 

The 20-year-old gunman, who had also fatally shot his mother in her bed, then killed himself. 

Previous coverage  

PARKLAND — The Latest on a shooting at a Florida high school (all times local):

6:30 p.m. 

A Florida sheriff says that 12 of the 17 confirmed deaths in Wednesday's shooting attack on a high school were found in the school.

Broward County Sheriff Scott Israel says the attack began outside the school Wednesday afternoon. 

He told reporters that authorities subsequently found 12 people dead in the building and two more dead just outside the school and one more in a nearby street. Israel says two other people died later under medical treatment. 

Israel says the suspect, a 19-year-old former student of Marjory Stoneman Douglas High School, is in custody. He says the male suspect was checked out at a hospital after his arrest and is now being held at a secure location in a public building. 

6:05 p.m. 

A sheriff says 17 people have died in the shooting attack on a South Florida high school.

Sheriff Scott Israel of Broward County says the 19-year-old suspect is in custody and that investigators are beginning to "dissect" what happened in the attack Wednesday at Marjory Stoneman Douglas High School in Parkland. 

He says the suspect, a former student, was previously expelled for disciplinary reasons. 

Israel says the man had at least one AR-15 rifle as well as multiple magazines. 

He says most of the fatalities were inside the building though some were found fatally shot outside. 

5:40 p.m. 

A man says he watched as officers arrested the suspect in the shooting at a Florida high school, where authorities are reporting numerous deaths. 

Michael Nembhard told The Associated Press he was in his garage watching TV news coverage of the shooting when he heard a police officer repeatedly yelling, "get on the ground!" 

Nembhard says he looked up to see a teenage boy on the ground about 150 yards (meters) away with an officer pointing a gun at him. The officer stood over the boy until other officers arrived, handcuffed him and led him away. 

A federal official who spoke on condition of anonymity identified the suspect as Nicolas Cruz. The official says he wasn't authorized to discuss it publicly. 

Authorities say the suspect is a former student about 18 years old. 

5:20 p.m. 

A federal official has identified the Florida school shooting suspect as Nicolas Cruz. 

The official spoke to The Associated Press on condition of anonymity. The official says he had been briefed on the investigation into the shooting at the South Florida high school, but was not authorized to discuss it publicly. 

Authorities in Florida say the shooter opened fire at the school Wednesday afternoon, killing "numerous" people. The shooting sent frightened students running out into the streets and SWAT team members swarming the building. 

Authorities later announced that they had taken a former student, about 18 years old, into custody after locating him off the school grounds.

5 p.m. 

Broward County Sheriff Scott Israel says the shooting suspect is an 18-year-old former student at the South Florida school where the shooting erupted. 

He says the teen was arrested without incident after he was located off the school grounds in a nearby community. He didn't give details of when the suspect had attended the school. But the sheriffs says the suspect wasn't currently enrolled. 

"I don't know why he left," Israel said, briefing reporters Wednesday afternoon. 

He also says the shooter was outside and inside the school at points during the attack. He didn't elaborate. 

The sheriff says several SWAT teams have gone in during the afternoon and are clearing every building at the Parkland high school complex to ensure no other threat remains. 

He also says the FBI has stepped in and will begin processing what he describes as "horrific scene." 

Said the sheriff: "This is a terrible day ... This is catastrophic."

4:25 p.m. 

A school official says there are numerous fatalities from the high school shooting in South Florida.

Broward County Superintendent Robert Runcie says "There are numerous fatalities. It is a horrific situation." He added, "It is a horrible day for us." 

The Broward County Sheriff's Office tweeted Wednesday afternoon that "so far we have at least 14 victims." The tweet added: "Victims have been and continue to be transported to Broward Health Medical Center and Broward Health North hospital." 

The sheriff's statement didn't elaborate on the victims or their injuries. 

4:15 p.m. 

The White House has canceled its daily press briefing after a Florida high school shooting that sent students rushing into the streets. 

President Donald Trump has spoken with Florida Gov. Rick Scott about the shooting. He says in a tweet that the White House is "working closely with law enforcement on the terrible Florida school shooting." 

He earlier tweeted his condolences to the families of the victims. 

Spokeswoman Sarah Huckabee Sanders says Trump has offered Florida federal assistance, if needed. The homeland security secretary has also been in touch with state and local officials. 

Sanders says, "We continue to keep the victims, and their friends and family, in our thoughts and prayers." 

Authorities say the shooter at a South Florida high school is now in custody. 

The Broward County Sheriff's Office gave no details in briefly tweeting that development. It did not identify the shooting suspect nor say how the person was taken into custody. 

Television footage showed police putting a person in the back of a police car outside the high school. 

4 p.m. 

Parent John Obin says his son, a freshman at the South Florida high school where the shooting erupted, says his child was in class when he heard several shots. The father says his son advised that teachers quickly rushed students out of the school. He adds the boy told his father that he walked by two people on the ground motionless — and apparently dead — as students rushed outside.

"This is a really good school, and now it's like a war zone," Obin said. 

Coral Springs Police said on their Twitter account Wednesday that Marjory Stoneman Douglas High School was locked down and that students and teachers inside should remain barricaded until police reach them. 

Outside, televised news footage showed two people on stretchers and another person being treated on the ground at an intersection near the scene of the school. Paramedics were treating those who appeared to be students with injuries, but it wasn't clear how they were hurt. A few students were loaded into ambulances. 

One unidentified student told a reporter at the scene that at first students thought it was a fire drill because they had heard fire alarms going off. 

3:55 p.m. 

The shooting at a South Florida high school sent students rushing into the streets as SWAT team members swarmed in and locked down the building. Police were warning that the shooter was still at large even as ambulances converged on the scene and emergency workers appeared to be treating those possibly wounded.

Aerial television news footage showed police in olive fatigues, with weapons drawn, entering the Marjory Stoneman Douglas High School in Parkland. Then dozens of students could be seen frantically running and others quickly walking out. A police officer waved the students on, urging them to quickly evacuate the school.

Some students exited the building in single-file rows with hands raised overhead to show they carried no weapons. Others held onto other students as they made their way out past helmeted police in camouflage with weapons drawn.

Emergency medical personnel pulled stretchers from the backs of ambulances as police cars surrounded the parking lot. At least one person was seen being wheeled to the ambulance on a gurney. It wasn't immediately clear how many people were wounded.

3:45 p.m.

Len Murray's 17-year-old son, a junior at the South Florida high school where shooting was reported, sent his parents a chilling text: "Mom and Dad, there have been shots fired on campus at school. There are police sirens outside. I'm in the auditorium and the doors are locked."

Those words came at 2:30 p.m. Wednesday. A few minutes later, he texted again: "I'm fine."

Murray raced to the school only to be stopped by authorities under a highway overpass within view of the school buildings in Parkland. 

No information was immediately given to parents, Len Murray says. And he says he remained worried for all those inside. 

"I'm scared for the other parents here. You can see the concern in everybody's faces. Everybody is asking, 'Have you hard from your child yet?'" 

3:15 p.m. 

Authorities say a shooter at a Florida high school is still at large. 

The Broward Sheriff's Office shared the information on its Twitter account after Wednesday afternoon's shooting at Marjory Stoneman Douglas High School in Parkland. 

It wasn't immediately clear how many people were wounded. 

2:30 p.m. 

Authorities say they're responding to a shooting at a Florida high school. 

The Broward Sheriff's Office has told news outlets the shooting occurred Wednesday afternoon at Marjory Stoneman Douglas High School in Parkland. 

It wasn't immediately clear how many people were wounded.