Letter being delivered (illustrative only)
Letter being delivered (illustrative only)Reuters

Police evacuated the U.S. Embassy in Stockholm on Wednesday after staff received a letter with “unknown content.”

Inside the envelope was “an unidentified white powder,” according to Swedish media. Emergency services and police officials were on the scene, witnesses said. Staff members were allowed to remain on the property, although they were told to evacuate the building.

Bomb disposal experts were also sent to the embassy as a precaution, police said.

Stockholm Police Albin Naverberg said , “I can confirm that there was a white powder in the envelope. It will be analyzed today or tomorrow, but for now no one knows what it is,” he told The Local.

Police sealed off streets around the area leading to the embassy as a precaution but were reopened after the envelope was removed. Some 150 people work at the U.S. mission, but it is unclear how many of them were in the building at the time.

"There has been a potential security incident that we are investigating and we have temporarily been evacuated,” said Jeff Andersson, embassy spokesperson. “We are working with Swedish authorities to investigate the incident,” he added. He added that there are no known threats directed against the embassy.

Suspicious packages containing white powder were sent to various locations around the United States, including government offices, in 2010, causing entire buildings to be shut down for security checks.

Each time, the substance was sent to laboratories for analysis to make sure the powder was not an explosive material or hazardous in any way as a radioactive, biological or chemical agent. Testing generally takes from 24 to 48 hours to rule out most risk factors.