ROYAL OAK, Mich. (AP) _ A bomb inside an envelope addressed to ''tax thieves'' exploded outside a post office Monday night, injuring a worker taking last-minute federal tax returns, authorities said.

The worker, whose name wasn't released, was taken to William Beaumont Hospital suffering from burns on his hands and face, said city Fire Chief Bill Crouch.

The 29-year-old victim was treated and released, said hospital spokeswoman Valerie Mabaczek, who declined to discuss the man's injuries. ''He asked that no information be given out.''

The blast occurred around 8:30 p.m. outside the post office in this Detroit suburb as workers collected tax returns.

Workers spotted the envelope, which had been placed in a bin, smoking. The victim had picked it up to put it in a puddle when it went off, Crouch said.

The envelope was addressed to ''tax thieves'' and contained at least one tea bag, the fire chief said.

The blast did little or no structural damage to the post office, said police Lt. Ray Taylor.

The FBI, the federal Bureau of Alcohol, Tobacco and Firearms agents and the state police bomb squad responded to the bombing, Taylor said. The bomb squad was called out to make sure there were no other bombs.

No suspects were in custody, Taylor said.