PARKLAND, Fla. (AP) — An orphaned 19-year-old with a troubled past and his own AR-15 rifle was charged with 17 counts of premeditated murder Thursday morning following the deadliest school shooting in the U.S. in five years.

Law enforcement officials told The Associated Press that Nikolas Cruz legally purchased the assault weapon used in the attack.

As reactions poured in Thursday, President Donald Trump focused on the young man's mental health.

"So many signs that the Florida shooter was mentally disturbed, even expelled from school for bad and erratic behavior. Neighbors and classmates knew he was a big problem. Must always report such instances to authorities, again and again!" Trump tweeted.

Fourteen wounded survivors were hospitalized as bodies were recovered from inside and around Marjory Stoneman Douglas High School.

Cruz, still wearing a hospital gown after being treated for labored breathing, and weighing in at 5-foot-7 and 131 pounds, was ordered held without bond and booked into jail.

His former classmates thought they were having another drill when a fire alarm sounded, requiring them to leave their classrooms Wednesday.

That's when police say Cruz, equipped with a gas mask, smoke grenades and magazines of ammunition, opened fire with a semi-automatic weapon, killing 17 people and sending hundreds of students fleeing into the streets.