A packet that aroused the suspicion of a letter carrier yesterday when she found it in a mailbox in Dumfries proved to be a letter bomb, authorities said.

The carrier took the device, measuring 6 by 8 by 1 inch, from the mailbox on Route 1 in front of the Dumfries Shopping Center to her post office on nearby Main Street, postal officials said. There, an X-ray confirmed that it was a bomb "designed to initiate when the recipient opened the parcel," said Paul Giroux, a spokesman for the U.S. Postal Inspection Service.

The bomb was later detonated in a parking lot behind the post office.

Postal officials would not identify the letter carrier, who they said made the discovery on her rounds about 11 a.m. Nor would they specify what alerted her. "Certain indicators . . . basically they aroused her suspicion," Giroux said. He would say only that the bomb was addressed "to an individual," whom he would not identify, in the Washington area.

Authorities said there appeared to be no political or racial motivation.

"It was no judge, no politician, no chief of police," said Dumfries Police Chief Conrad G. LaBossiere. "More than likely it was probably domestic, but until the investigation is completed by the Postal Service, we can't confirm that."

Giroux said there appeared to be no connection between the Dumfries incident and a mail bomb that exploded in June, injuring an Alexandria psychiatrist in his office, and a device that exploded in December injuring a Circuit Court judge in Washington County, Md. Both of those cases are still under investigation, he said.

Postal Service investigators called the Marine Explosive Ordnance Disposal Team at the Quantico Marine Base, which set off a charge to safely explode the bomb behind the post office about 5:30 p.m.

Explosives experts chose to set off the charge, instead of opening the package, because "if somebody was opening it, it could have been quite destructive," Giroux said.

Police said the bomb yielded some clues to the identity of its builder, but there were no suspects.