The Arvada, CO, City Hall was evacuated and a bomb squad called in after a car carrying at least one propane tank was parked near the building and the driver ran away, officials in the Denver suburb said.
The driver left the car near an entrance leading to police headquarters and fled carrying a duffel bag, said city spokeswoman Maria VanderKolk.
VanderKolk said the driver of the vehicle was at large and authorities were treating the situation "very, very seriously." She said she did not know whether the car contained other devices or wires.
VanderKolk said the car was discovered shortly before 7 a.m., when only a handful of people were in the building, mostly in the police offices. She did not know how many people were evacuated.
No homes were evacuated, but nearby residents were alerted by phone, police spokeswoman Susan Medina said.