A 19-year-old former student with a history of menacing social media posts was booked into Broward County Jail Thursday morning on charges of premeditated murder in the killing of 17 students and faculty at a Florida high school. Nikolas Cruz, expelled last year for fighting, returned to Marjory Stoneman Douglas High School in Parkland, Fla., with a vengeance Wednesday, bolstered by a gas mask, smoke grenades and multiple magazines of ammunition and a semi-automatic weapon.

He was moved to jail early Thursday from a hospital, where he was initially taken for labored breathing after his arrest. He was in handcuffs and wearing a hospital gown when he was transported before dawn. It was the nation’s deadliest school shooting since a gunman attacked an elementary school in Newtown, Conn., more than five years ago. Cruz, who is being held without bond, tried to slip away after the carnage by blending in with fleeing students, but was recognized by fellow teens who knew him for his quick temper and erratic behavior.

The 17 dead include students and adults, Broward County Sheriff Scott Israel said. Fourteen were injured and taken to local hospitals. A sheriff said the bodies of 12 of the dead, including a beloved coach and security guard, Aaron Feis, who stepped in front of one spray of bullets to protect his students, were found inside the building. Counselors were made available Thursday morning off-campus for the more than 3,000 students, teachers and staff at the school, which will remain closed as an investigation continues.