The suspect, believed to be about 18 years old, is a former student at Marjory Stoneman Douglas High School in Parkland, where there were at least 20 victims, Broward County Sheriff Scott Israel said.

It wasn't immediately clear how many of the victims were injured and how many had been killed. Sen. Bill Nelson, D-Fla., citing Broward County officials, told NBC News that there were "a number of fatalities."

Several students told NBC News that the school had gone through a fire drill earlier in the day. They said the fire alarm sounded again just before the shots were heard.

Relieved parents like Lisette Rozenblet, whose daughter attends the school, also said she was told that a fire alarm was pulled about the time the shots began. But her daughter's teacher, sensing that it might be a trap, told the students to stay in the classroom, she said.

Thoughts and prayers are with everyone involved.

Stay tuned for updates.