STOCKHOLM, Sweden -- Police say the U.S. Embassy in Stockholm has been evacuated because of a letter with "unknown content."

Stockholm police spokeswoman Tove Hagg said Wednesday that staff were evacuated from the building but remain on the premises. Streets around the embassy were closed.

She couldn't give any details on the content of the letter. Swedish tabloid Expressen, citing firefighters, says on its website that a "white powder" had leaked from the letter.

Hagg says police bomb disposal experts were sent to the embassy as a precaution.