The gunman reportedly had a rifle and possibly a handgun as he entered the school on Wednesday afternoon, before opening fire.

Broward County Sheriff Scott Israel told media that the man was a former student of the school. It’s unclear what caused him to leave or when it happened. Israel described the suspect as a man about 18 years old. He was apprehended off-campus about an hour after the shooting unfolded, Scott said, adding that the teenager did not attempt to resist the arrest.

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The attacker has been identified as 19-year-old Nicolas Cruz, according to AFP and CBS News reports, citing officials with the knowledge of the investigation.

Israel said the shooter was active inside and outside the building, without providing any details. CBS News reported that prior to the attack Cruz triggered the fire alarm, which caused panic.

Witnesses’ reports appear to confirm this timeline. One of the students who was locked inside during the incident told CBS News that the school had already conducted a fire drill in the morning, so the students were surprised at what they thought was another one.

The suspect reportedly attempted to flee the scene by blending in with other students.

His motives are unknown so far.