US embassy staff in Stockholm returned to their workplace on Wednesday after a suspect envelope prompted an evacuation earlier in the day.
"The personnel has been allowed to go back to the embassy," embassy spokesman Jeff Anderson said.
Staff and members of the public had been evacuated during a "preliminary investigation", he said.
Swedish police said on their website that a patrol had been sent to the embassy after "a letter with unidentified contents had arrived at the embassy".
"It's a substance one would not expect to receive," Stockholm police spokesman Albin Neverbery said.
News agency TT said the letter contained an unidentified white powder, but police would not confirm the report.
"When it comes to a sensitive location like an embassy, we take no chances," Neverbery said, adding that police had no information about any threats issued against the US mission.
"A police bomb squad has taken care of the envelope. We're securing the site."
The US mission in Stockholm has some 170 employees.