CBS Philly is reporting that FBI agents have joined Philadelphia police in investigating an explosion on a bridge passing over the Amtrak train tracks in the city’s Brewerytown neighborhood. Authorities have determined that the blast was not due to an exploding transformer, as originally suspected.

Mayor Michael Nutter’s press secretary has tweeted that two persons of interest have been hospitalized following the blast, which left a 2-foot crater. reports:

“The bomb squad was called and determined that … there was an explosion of some kind creating a roughly two-foot deep crater,” Mayor Michael Nutter’s spokesman Mark McDonald said. “Police radio learned there were two males that were picked up by EMS in the area of the bridge and taken to Saint Joseph’s Hospital with minor injuries, scraps and cuts, injuries that possibly you might see with an explosion.”

* * *

Update: Philadelphia police now say the explosion was a freak accident.