Briseno had rushed to help after the shooter’s pickup crashed outside Robb Elementary School, but fled after seeing the 18-year-old with an “evil look” holding a rifle.
In an exclusive interview with NBC News, Briseno, who has helped bury several of the victims, including a relative, described the encounter and how he was prepared to charge the shooter after his wife brought him his gun, but was told by arriving officers to back away.
“Hey, what are you doing,” an officer asked as Briseno began to march toward the school, he said. He recalls responding, “I’m going to go in and try to stop them.”
As police who responded to the 77-minute attack, which ended with 19 children and two teachers dead, face criticism about the delayed confrontation, they’re also being admonished by some city residents, including parents who rushed to the scene, for preventing them from going in.
“I told him that he’s already inside the school,” Briseno said. He said the officer told him to stay back and shut up.
Briseno did not want to say which department employs the officer.
Texas Department of Public Safety Director Steve McCraw, whose officers responded alongside school police, city officers, county sheriff’s deputies, and federal agents, has said the delay in response was the “wrong decision.”
Briseno said he has spoken with investigators, and a search warrant affidavit also includes details of what he and his coworker told Texas Rangers.
Briseno told NBC News he was with his coworker when they heard the pickup crash outside the campus. They approached within about 8 feet to check on the driver when he emerged.
“I see him crawling out from the passenger window,” he said. “I tell them, ‘Hey, man, are you OK? Are you all right?’”
“We locked eyes, and he gave me this vibe,” Briseno said. “At that moment, he looks right back to me ... with that evil look, and I see is this rifle.”
The gunman, who was ultimately killed by U.S. Border Patrol tactical agents, grabbed what Briseno described as an AR-15 and started to raise the weapon, he said.
The pair ran, and Briseno tripped on a curb and fell. He said he saw the man attaching a magazine to the weapon before the sound of gunfire and ricocheting bullets filled his ears.
“I get up, and as I’m running, I look back,” Briseno said. “And he was aiming that barrel right at me and my coworker.”
Another pair of workers at Hillcrest Memorial Funeral Home held its front doors open for the running men, and locked them after they arrived, he said.
Briseno looked outside to see the gunman kneeling and reloading and then heading for the school. He phoned his wife: “Hey, bring me my gun,” he says he told her.
Gunfire was erupting at the school, he said. He says he saw the man shoot into campus windows.
By the time his wife arrived, the first officers were arriving, too, Briseno said. He marched ahead, gun in hand, when he was stopped by police.
“I feel guilty man, ‘cause I couldn’t stop (him),” Briseno said emotionally. “He was shooting at the windows, and I didn’t have my gun on me.”
Briseno said he wants the families to have closure and hopes that sharing what he witnessed will also help investigators.
One of Briseno’s main duties at the funeral home is digging graves. He said he’s helped bury five children who died that day, including a cousin, Amerie Jo Garza, whose service was Tuesday.
“Had to dig the grave, set up the tent, lower her casket,” Briseno said.
“It always plays in my head,” he said, when “I loaded those caskets down.”
This story first appeared on NBCNews.com.