Pittsburgh police are searching for a white truck in connection with an incendiary device thrown from a vehicle on Sunday night. A second explosion was reported in the city a short time later.
The first event took place just after 9 p.m. on Penn Avenue in the city's Lawrenceville area. The device was thrown from a moving vehicle and landed under a parked car, as NPR member station WESA reported.
No one was injured, but the car was damaged.
A witness told the Pittsburgh Post-Gazette that the explosion was "not much, just a loud boom around 8:50, then sirens."
Ninety minutes later, police received reports of an explosion about two miles away in the city's Hill District. Police said that residents "reported feeling their homes shake and some type of odor in the air."
The Pittsburgh Police Bomb Squad brought bomb-sniffing dogs to the site, but so far no evidence has been found in what police called a "possible explosion" in an update Monday morning.
Police said they are looking for a white two-tone older model pickup truck in connection with the first explosion, and are still reviewing footage in the second instance.
The events in Pittsburgh come a little more than a week after the explosion of an RV rocked downtown Nashville on Christmas morning.
The man suspected of carrying out that bombing was killed in the blast. Three people were hospitalized with injuries, and dozens of buildings were damaged.