MADISON, Wis. -- Dane County employees return to work on Tuesday to the same downtown Madison parking ramp where a pipe bomb exploded on Christmas Eve.

The ramp is located at Main and Henry streets, WISC-TV reported. A majority of the government employees use the ramp at Doty and Fairchild.

The FBI is now investigating the explosion, which occurred off shortly after noon on Christmas Eve, when a man returned to his car to pay the meter.

The bomb had apparently been placed on the tires of a car. When the driver returned and leaned up against the car, the bomb went off, WISC-TV reported.

The man suffered only minor hearing damage, but luckily, no other injuries.

There were only a few cars in the ramp because of the holiday weekend, WISC-TV reported.

This is the third time a bomb has been found in the same ramp since November, but the first time that one detonated.

Madison police said that they wouldn't speculate on whether the three incidents are related.

Madison police spokesman Howard Payne said that the other two objects looked like bombs but turned out to be harmless.

Payne said that his department is collaborating on the investigation with the FBI and the federal Bureau of Alcohol, Tobacco and Firearms. He said that they're concerned about the recurrence of the incidents.

Some county workers in the ramp on Monday said they are being very cautious.

"I'd definitely keep my eyes open, that's for sure," said Jason Kelter. "I think everyone should if they're going to be parking here. I'm going to keep an eye open for any suspicious activity because you never know these days what is going to happen."

Madison police said that there are no plans to beef up security in the ramp until it receives more information from the FBI.

Authorities who manage the parking ramp said that people parking there shouldn't panic, but make sure they lock their car doors and avoid walking alone in the ramp.