A family of three, including a two-year-old child, escaped injury when a device exploded near their car in Lisburn on Friday.
Minor damage was caused to the car when the pipe bomb exploded at Brokerstown Road at 0930 GMT.
The area has remained closed while police carry out a search.
Police said it was not clear whether the device exploded as the car drove past it or over it, or whether the device fell off the vehicle.
A PSNI spokesperson said initial police inquiries indicated that the family had no connection with the security forces.
Police are asking anyone who saw anything suspicious in the Brokerstown Road area in the 24 hours before the explosion to contact them.