STOCKHOLM: Police have seized a letter which arrived at the U.S. embassy in the Swedish capital on Wednesday for examination and begun to allow people to return to the building after it was evacuated, officials said.

Everyone apart from essential staff had earlier left the building, where about 150 people are employed, after a letter arrived with a mysterious substance inside.

A fire service spokeswoman had said that police had told her a white powder had leaked in the building.

"We have lifted the barriers and seized this letter, which is being examined as part of a preliminary investigation into into unlawful assault," said a spokeswoman for Stockholm police.

She declined to go into details of what the letter contained.

People were now allowed to return to the building, she said.

Police had earlier sealed off an area around the embassy, which is a wealthy part of Stockholm, where other countries have their diplomatic representations.