Two packages detonated in the mailrooms of two Maryland state government buildings on Thursday, but no one was seriously injured, officials said.
Investigators had yet to identify a suspect or make any arrests, a Maryland state official said.
"Explosion may be too strong of a word. The packages were smoldering. In one case there was a flare up," said the state official, who asked not to be identified because Maryland State Police was in charge of the investigation.
There were no serious injuries, though a few people were examined by emergency medical technicians, the official said.
A federal law enforcement official said the packages erupted into smoke and flames. No one was taken to the hospital.
One incident took place in the mailroom of the Maryland Department of Transport facility in Hanover and the other in a mail-handling facility in a state building in Annapolis next to the Maryland State House.
Authorities evacuated about 125 people at the Jeffrey Building and some 250 people from the Department of Transportation building, the state official said.
(Reporting by Jon Hurdle, James Vicini and Anthony Boadle, editing by Daniel Trotta and Sandra Maler