Santa Fe High School had conducted active shooter drills, armed police officers patrolled the hallways and students went through a scare in February after a false report of a campus gunman.

But in the aftermath of the deadliest public school shooting in Texas history, early witness accounts and recordings from emergency dispatch describe a 30-minute nightmare as the real thing unfolded last week, even as authorities continued to keep details close Sunday.

Among the biggest unknowns is when the confrontation began at the high school outside Houston between police and 17-year-old Dimitrios Pagourtzis, who authorities say opened fire on an art lab with a shotgun and .38 caliber handgun shortly after the first bell Friday morning. Pagourtzis wasn’t hit in the attack even though officials have described him engaging in a drawn-out firefight with police.

Ten people were killed, most of them students. Galveston County District Attorney Jack Roady declined to answer questions about the shootout and investigation Sunday, including whether police may have hit any students in a gunfight with the shooter.

He also said autopsy reports won’t be released while the case is pending.

The length of the attack appeared to go on longer than most mass shootings that last around eight minutes, said Ben Tisa, a former FBI agent. He said that could make the Santa Fe shooting unusual.

“It would be unless they couldn’t find the guy and they had to hunt him,” said Tisa, who now does tactical training in California.

Although officials have praised a swift response, it remains unclear just how quickly police got to the art lab on the 1,400-student campus. Galveston County Judge Mark Henry, the county’s top administrator, has said police exchanged rounds with Pagourtzis “for quite a while” before he surrendered a half-hour after the first reports of a shooter on campus.

“They said there was a lot of firepower and a lot of rounds exchanged,” Henry said.

Officials have not yet released 911 tapes but on emergency dispatch recordings from Galveston County, captured by Broadcastify.com, a female voice is heard saying “more shots fired” about 10 minutes after authorities first received reports of gunfire. Five minutes later, a male voice says the suspect is “possibly going to be barricaded” with additional reports of shooting a few minutes after that.

“He’s actively shooting. He’s in the art room. We’ve got, we’ve got shots fired right now. We need you all up here,” a male voice says at what appears to be about 15 minutes after the shooting began.

Henry said investigators were still working on the timeline and Tisa cautioned that emergency dispatch traffic doesn’t always reflect real time. One Santa Fe school police officer who responded to the attack was shot and remained in critical condition Sunday, according to the University of Texas Medical Branch.

Zach Wofford, an 18-year-old senior, was across the hallway when the shooting began and said he heard gunfire that lasted 10 to 15 minutes from the art classroom. That’s where Breanna Quintanilla, a 17-year-old junior, was when the attack began. She said Pagourtzis had aimed at her and missed but that it ricocheted into her right leg.

She recalled the voice she heard after the first sound of gunfire in the class: “If you all move, I’m going to shoot you all.”

(AP)