Three men were killed when a car exploded in Pennsylvania's third largest city Saturday night, drawing a massive response from local and federal law enforcement to what was dubbed a "criminal incident."

At a news conference Sunday afternoon, authorities repeatedly described the explosion in Allentown as an "isolated incident." Lehigh County District Attorney Jim Martin said investigators had a "high degree of confidence that the perpetrator was probably killed."

There was no immediate word on the identities of the victims, nor was there any explanation of a possible motive or cause of the blast. Investigators said they did not believe there was any additional threat to the community.

The explosion happened at approximately 9:30 p.m. on a downtown street and could be heard and felt across the city located about 60 miles northwest of Philadelphia.

At least one person was killed after a car exploded in Allentown, Pa. on Saturday. (FOX29)

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

"The fire was crazy," Perodin told the paper. "The car was pretty much split in half."

Another witness, Stephanie Connelly, said she saw body parts strewn across the street.

"This is real life and I'm shaking and freaking out while I'm running," she told The Morning Call.

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 and authorities said it would remain open on Sunday night.

Authorities shut down roughly six blocks of downtown Allentown, Pa. as part of the investigation. (FOX29)

The federal Bureau of Alcohol, Tobacco, Firearms and Explosives is taking the lead on the investigation with the assistance of the FBI and local authorities. Officials have asked anyone with information about the case to call (888) ATF-BOMB (283-2662).

The Associated Press contributed to this report.