At 8:12 a.m. Tuesday, a caller alerted the UT Police Department to the presence of a masked gunman on campus. Seven minutes later, the dispatcher sent a text message warning 53,000 people, and the University siren sounded five minutes later. Shortly afterward, eight UTPD officers and two Austin police officers chased the man up the stairs to the sixth floor. At 9:53 a.m., police announced the gunman was dead on the sixth floor of the Perry-Castañeda Library.

UTPD Chief Robert Dahlstrom said the procedure for this sort of situation is simple — start the hunt and eliminate the threat before anyone gets hurt. UTPD sent every officer they had on hand to respond.

“Once they got on the sixth floor, they found him fairly quickly,” Dahlstrom said. “They called it in, saying he was down.”

But UTPD couldn’t give the all-clear yet as rumors spread of a second gunman. The police department began setting up a perimeter with road blocks and electronically locked down 23 nearby buildings, sweeping the area with APD and Travis County SWAT, Texas Rangers and UTPD. An armored SWAT vehicle sat near the library as well.

The protocol for clearing the campus of additional unknown shooters was clear: Establish a perimeter, search campus buildings and keep students in classrooms until their buildings were cleared.

Students were told to barricade their classrooms.

In Calhoun Hall, engineering sophomore Mark Divalerio sat with other students barricaded for two hours in his classroom. The second shooter was thought to be in the area of the building, so when a student in the hallway tried to open the classroom door, they almost panicked, Divalerio said.

“They tried to open up the barricaded doors; that was scary, but other than that we just turned out the lights and hung out,” he said.

Eventually, the rumors subsided and the police departments decided the campus was safe.

“We had information that possibly there was another shooter, and that just comes from the excitement, but we can’t just say, ‘Oh, that’s all it is,’” Dahlstrom said. “We have to check it out until we feel very comfortable.”

A significant number of agents from the FBI arrived on the scene, helping local authorities as a matter of practice, said FBI spokesman Erik Vasys. The FBI is currently helping the police departments shed light on the shooter’s background. In Tuesday’s shooting response, Vasys said the FBI personnel were present mostly in case UTPD or APD officers tired out.

“We didn’t have to put forth a tactical response, but we had one prepared if they needed backup,” he said. “A lot of times manpower gets worn out from the heat when something turns out to be long-term, so we’ll back them up.”