ALLENTOWN (AP) — Investigators returned to the scene Monday of a car explosion that killed three people in downtown Allentown, trying to get to the bottom of the deadly blast that authorities were treating as a crime.

About 30 investigators combed the scene, most of them from the federal Bureau of Alcohol, Tobacco, Firearms and Explosives. Some wore hazmat suits.

Three males were found dead after the 9:30 p.m. Saturday blast, according to Lehigh County Coroner Scott Grim. Autopsies were planned for Monday, and authorities planned to release their names pending confirmation of their identities and notification of their families.

The perpetrator was believed to be among the dead.

Officials said the blast was believed to have been an isolated incident with no ongoing threat to the public.

“We know there’s been a criminal incident,” District Attorney James Martin told reporters at a news conference Sunday. “We have a high degree of confidence that the perpetrator was probably killed in the incident.”

Authorities released no information about the possible cause of the blast, including whether it was a bomb, but Martin said “loads of us in law enforcement” are confident “this was A. an isolated incident and B. there’s no continuing threat.”

Authorities are seeking the public’s help and asked anyone with information to call the ATF.

Resident Carlos Perodin told The Morning Call of Allentown that he was watching a movie with his wife when he heard a thunderous explosion and went to the scene.

“The fire was crazy,” he said. “The car was pretty much split in half.”

Stephanie Connelly, who saw the aftermath of the explosion, told The Morning Call that she saw body parts strewn across the street.

A bus station was turned into a makeshift command center with armored vehicles, dozens of police cruisers, mobile command units and even portable bathrooms, the paper reported. Several portable tents were also erected for evidence processing.

Residents were asked to avoid the area, and people who live nearby were asked to shelter in place. A shelter was set up at an elementary school.