Four people were hospitalized with minor injuries Friday night after an explosion on a bridge over the train tracks at 30th and Cecil B. Moore in Brewerytown.

The explosion was originally thought to have been an explosive device, but after an extensive investigation, police are calling it a "thermic event."

"I have good news to report. There was not an explosive device detonated tonight," said Chief Inspector Joseph Sullivan during an evening news conference.

Chief Sullivan says icicles formed as a result of the recent moisture and these icicles came in contact with high power lines from AMTRAK that run underneath the bridge.

"There was a pressure wave that came up through bridge, created a debris field, all the indications of an explosive device having detonated," said Sullivan.

Two K-9 dogs brought to the scene were not able to detect any signs of an explosive device, so investigators called on an AMTRAK electrical engineer, who surveyed the bridge.

"An explosion of this magnitude is very unusual, but an event of this nature is not," said Sullivan, who added that the explosion blew large chunks of concrete over quite a far distance. He estimated there's now a hole approximately two feet wide on the bridge.

Two of the injured people were hurt when debris hit their car as they were driving by.

Service was temporarily suspended on the Chestnut Hill West and Trenton Lines during the investigation.