A powder found in a mail bag shut down the post office here for about six hours, until tests showed it was not harmful, postal officials said.

Police and postal officials said the powder was found about 11 p.m. Sunday in a mail bag.

"It was in a place where we couldn't determine exactly what piece of mail it might have come out of," Postal Service spokeswoman Teresa Rudkin said.

Rudkin said the building reopened at 6:30 a.m., after tests showed the powder was not dangerous. Mail was delayed.

"We just followed protocol," Rudkin said.

She did not know what the powdered substance was. It was sent to a lab for tests.

"They just tell you want it isn't," Rudkin said.

The Bismarck post office was the only one affected, she said.

In March, a suspicious powder that leaked from an envelope at the Jamestown post office was found to be drugs.

Post offices in Bismarck, Fargo and Jamestown were closed for about 21/2 hours until tests ruled out anthrax.