County dispatchers said crews were called to the 600 block of Third Street around 9:12 a.m., just a few hours after polls opened for the primary election.
There was some "sort of explosion" near the vehicle about a half-block away from Salem United Church of Christ, which is a polling place, Trooper Nathan Branosky said.
State police say nobody was injured and there is no threat to the public.
Chopper 6 over investigation after unknown device thrown from vehicle
The motive remains unknown. State police say there is no evidence so far to believe the incident was related to the polling location.
People who live and work nearby say whatever it was, it was loud.
"I heard an explosion, wondered what it was, came running outside to see a puff of smoke in the middle of the intersection," said Aaron Rigo.
"I was getting ready to leave the house to come down and vote and I heard 'boom,' and I thought my basement pulled up," said Sharon McKee.
Investigators were seen door-to-door interviewing neighbors, and looking for possible surveillance video of the incident but, so far, no suspects have been named and no arrests have been made.
In the meantime, while the area remained closed off to vehicles and pedestrians into late afternoon, voters were still able to access the polls inside the church from a rear entrance.
Despite that, local election officials say all of this commotion appears to be having a negative effect on voter turnout.
"Prior to all of this happening, we were getting a steady stream of people in. Since this has occurred, and the streets are all blocked off, we're getting very few people coming in," said Louis Draxler, Catasaqua Judge of Election-3rd Dist.
Anyone with information is asked to contact PSP Bethlehem at 610-861-2026.
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WFMZ-TV contributed to this report.