Police spokeswoman Susan Medina said the owner of a car earlier determined to be "suspicious" had taken his girlfriend to the building around 7 a.m., but got tired of waiting for her, grabbed his duffel bag and ran off.
Medina says the man called police at 9:30 to explain, after seeing his car on live television news coverage.
But it took authorities until much later to confirm the man's identity, and his story. Medina says police checked out the man and the car, and that they believe his report.
The car had at least one large white cylinder in the back that authorities believed to be a propane tank.
The Jefferson County Bomb Squad and other law enforcement officers used robotic equipment and their own personnel to determine what, if any, danger the car and the tank posed.
Bomb squad members could be seen opening the hood, hatchback and all of the doors of the vehicle and attaching a pulley system in an attempt to lift the propane tank out of the car.
Postmaster Vince Fresquez said 70 workers were evacuated from the post office.