A 24-year-old man who shot three teenagers on a SEPTA bus Thursday is a University of Pennsylvania security guard, Philadelphia District Attorney Larry Krasner said Friday. No charges have been filed, and the investigation is ongoing, Krasner said.

Authorities have video of the shooting, about 1 a.m. Thursday on a Route 58 bus in Northeast Philadelphia, and a video-recorded statement that the man gave to police, Krasner said. It will take time to determine whether charges should be filed, he said during an online news conference.

A spokesperson for Allied Universal Security Services said in an email Saturday that the man was employed by Allied and was off duty at the time of the shooting.

“This Security Professional has been suspended from duty pending an investigation,” Allied spokesperson Vanessa Showalter said. She confirmed Monday that the man worked on Penn’s campus, but said she could not disclose his name for reasons of confidentiality.

Krasner said Friday: “We are continuing to work with the police on this investigation. Once we have come to specific conclusions about what to do, in light of the complications here, in light of the fact that we have three young people who were shot, and we have to make sure that we are fair to everyone ... this will take a little bit of time.”

“But I can tell you that we are looking very closely at everything,” Krasner said.

Police said the man told them that he was sitting in the back of the bus when the teens attacked him and that he pulled out his gun and shot them in self-defense.

Police found the boys with leg wounds just outside the northbound bus while it was stopped on the 7200 block of Bustleton Avenue in the Castor section.

A 16-year-old boy shot in his left thigh was taken by police to Jefferson Torresdale Hospital. A 15-year-old boy with gunshot wounds in his right thigh and right buttocks and a 17-year-old boy shot in his right knee were taken to Einstein Medical Center. All three were in stable condition, police said.

Lt. Dennis Rosenbaum of Northeast Detectives said Thursday afternoon that the shooter had a permit to carry a gun.

Three other passengers and the 35-year-old bus driver were unhurt.

Editor’s Note: This article was updated on Monday, April 20, to note that the security guard is an employee of Allied Universal Security Services.