PITTSBURGH — No good deed goes unpunished — especially one that only partially undoes a bank robbery.

That's why FBI agents in Pittsburgh are scouring an envelope filled with bubble-wrapped stacks of money mailed to a southwestern Pennsylvania bank agents believe is most of the money taken in a robbery there last week.

"We cannot definitively narrow it down that it was bank robbery loot," said Special Agent Jeff Killeen of the Pittsburgh FBI office, "but there's no other reason to return it."

Killeen said the money received Monday in the padded manila envelope at the PNC Bank in West Newton amounts to about 80 percent of the amount taken by a robber who handed a note to a teller, demanding money on Dec. 9. PNC Bank officials said they don't comment on robberies.

"This is definitely the first time I've ever seen, in more than 25 years of being with the FBI, someone give back money they've stolen," Killeen said, though he acknowledged the FBI has yet to prove it.

It's possible someone close to the robber returned the money instead. No matter, agents are examining the envelope and money for clues — though Killeen wouldn't say Thursday what tests are being run.

Among potential clues, FBI agents could seek fingerprints, perhaps find a hair shed by the person who mailed it, or even obtain DNA from whoever licked the stamps, Killeen said.

Yet investigators believe they're more likely to find the robber if somebody recognizes him in a series of surveillance photos released to the media.