Police investigators are trying to determine the source of a baking soda-packed envelope that was opened Monday morning at the downtown headquarters of the Historic Charleston Foundation.

The situation blocked East Bay Street near The Battery for about three hours as explosives and hazardous-materials experts tried to figure out what they were dealing with.

Tourists, delivery drivers and joggers were turned away, but a wet dog and a man taking a morning stroll managed to slip through the scene at 40 East Bay St.

The police did not immediately say who had opened the envelope, to whom it was addressed or how big it was.

Battalion Chief Randy Carter, who heads the Charleston Fire Department’s haz-mat squad, said crews tested the white powder with a laser.

It turned out to be “100 percent baking soda,” or sodium bicarbonate, Carter said.

The Charleston Police Department was called about 8 a.m. to the Historic Foundation, a three-story brick building along Charleston Harbor just north of Water Street, agency spokesman Charles Francis said.

After finding the envelope, police officers summoned firefighters.

Police and fire crews blocked the road with cars and trucks. Two firefighters donned white suits, masks and air tanks, as a police officer strapped on armor. But they never were seen walking into the building.

About 20 workers inside the Historic Foundation were “sheltering in place” during the ordeal, Carter said.

Around 11:30 a.m., a test that took only four minutes determined that the powder was baking soda, he added.

Workers then walked in and out of the building, and tourists were allowed to mosey toward The Battery and snap pictures of the stately homes south of Broad Street.

Reach Andrew Knapp at 937-5414 or twitter.com/offlede.