SAN BRUNO -- A letter leaking white powder -- later ruled harmless and possibly a prank -- prompted a partial evacuation Tuesday of Walmart's online commerce headquarters, authorities said.

A worker discovered the powder seeping from a letter in the first floor mailroom at the building at 850 Cherry Ave. around 12:20 p.m. Testing subsequently showed it to be baking soda, but authorities were still investigating why it was sent to Walmart.com's offices, said Central County, Millbrae and San Bruno Deputy Fire Chief David Downing.

Downing said there's no good reason to send a powder-filled letter and authorities were looking into whether it was a prank or the work of an unhappy worker. Investigators were following leads, including the Redwood City return address on the envelope.

While fire crews worked at the building the first floor was evacuated as a precaution, which forced at least 20 workers out of the office. No one was injured, Downing said.

After discovering the powder, a worker took the letter outside, sealed it in a container and called for help. The hazardous materials team investigated and then used the county mobile lab to test the powder, Downing said. Firefighters were on Cherry Avenue for over 2½ hours.

Contact Joshua Melvin at 650-348-4335. Follow him at Twitter.com/melvinreport.



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