Surveillance footage of the Robb Elementary School shooting, published by the Austin American-Statesman and KVUE on Tuesday, surfaced ahead of a planned release by officials on Sunday.

The disturbing footage shows the gunman, 18-year-old Salvador Ramos, firing multiple shots from an AR-15-style rifle outside the school, then entering the building at 11:33 a.m. and shooting his way into adjoining classrooms.

More than 100 bullets were fired inside those two classrooms over the next two-and-a-half minutes before seven officers ran into the school at 11:36, two of them armed with rifles and four others armed with handguns.

WATCH NEVER SEEN BEFORE SURVEILLANCE FOOTAGE:

The gunman appeared to fire from inside the classroom at 11:37 at the officers, who retreated back down the hallway.

According to the footage, Ballistic shields arrived at 11:52 a.m. and 12:04 p.m., along with other officers armed with rifles, bulletproof vests, helmets, and other tactical gear.

At 12:21 p.m., officers walked down the hallway to position themselves closer to the classroom doorway. At 12:30, an officer can be seen walking over to a hand sanitizer dispenser and disinfecting his hands.

The classroom door was finally breached at 12:50 p.m., 74 minutes after the first officers entered the school.