Coulson said the St. Joseph's County Sheriff's Department is leading the investigation.
"We're assisting in any way we can," Coulson said, referring to ATF.
Mullen is running in Indiana's 2nd congressional district. His spokesperson Andy Reynolds told TPM the incident occurred between 4:45 p.m. and 5 p.m. on Monday. Mullen, his campaign finance director, and an intern heard the explosion, and went outside to see what had happened. Several neighbors also left their homes in response to the noise. According to Reynolds, both Mullen and his financer director spotted a black car driving away "out of corner of their eye[s]."
Reynolds said that Mullen then "spoke with the police, they came and checked it out," and that local authorities later contacted the ATF and the FBI. But he noted that at "this point it's more of a precautionary measure."
"We certainly hope that it's not a politically motivated thing," Reynolds said.
Calls to the St. Joseph County Sheriff's Office and the FBI were not immediately returned.