Two-year-old Jonathan Schmoyer and father Jacob Schmoyer were killed in Saturday night's explosion. (Facebook)

The three people killed in a car explosion in Allentown, Pa., were identified Monday as a father, his toddler son, and an elderly man who was a friend of the father.

Lehigh County Coroner Scott Grim said 66-year-old David Halman, 26-year-old Jacob Schmoyer and 2-year-old Jonathan Schmoyer were killed in the Saturday night blast that rocked Pennsylvania's third-largest city. The cause of the explosion remains under investigation.

Donald Robinson, a special agent in charge with the federal Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) told reporters that Halman and the elder Schmoyer were friends, but declined to comment further on their relationship.

Lehigh County District Attorney Jim Martin described the explosion as a "criminal incident" Sunday and said that that authorities were confident that the perpetrator was killed in the explosion. However, Robinson of the ATF said Monday that the explosion "could be anything from an accident to a device, an explosive device." It was not clear whether the ATF believed either Halman or Jacob Schmoyer had purposely caused the explosion. Martin did not speak at Monday's news conference.

Investigators returned to the area of the explosion early Monday with about 30 of them combing the scene, most of them from the ATF. Some wore hazmat suits.

Robinson said agents had removed three cars to continue examining evidence, but said the scene likely would be cleared later Monday.

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

The Associated Press contributed to this report.