Austin Wyatt Rollins, the 17-year-old who opened fire on classmates at Great Mills High School in Southern Maryland last week, injuring one and killing another, died by shooting himself in the head, officials investigating the case said Monday.

According to details released by the St. Mary's County sheriff's office, Rollins parked his car at 7:50 a.m. and walked through the school's main entrance two minutes later, reports the Baltimore Sun.

At 7:57 a.m., he approached classmate Jaelynn Willey, 16, and shot her once in the head with his father's Glock 9-millimeter gun. That bullet also struck 14-year-old Desmond Barnes in the leg.

After firing the gun, Rollins kept walking through the school, where he was confronted by school resource officer Deputy First Class Blaine Gaskill just after 8 a.m. Their weapons went off simultaneously 31 seconds later, with Rollins shooting himself in the head and Gaskill shooting Rollins in the hand, officials said.

The shot Rollins fired was fatal, according to the sheriff's office.