The embassy was evacuated late on Wednesday morning when an envelope containing a mystery white powder arrived at the Stockholm building.
Police and explosives experts were called in and the area was cordoned off until the package was removed. The staff and members of the public were allowed to return shortly after 2pm.
Later that afternoon, police were able to confirm that the powder was poisonous:
“I cannot discuss exactly what it is right now, but it has been determined that it is extremely toxic and corrosive,” said Fredrik Nylén of the Norrmalm police to the Expressen newspaper at the time.
Now, Säpo has taken over the investigation, although the service is yet to determine exactly what the substance is or why it was sent.
“We haven’t had any results to our tests yet,” said Sara Kvarnström, Säpo spokesperson, to the TT news agency on Monday.
Meanwhile, Jeff Andersson, spokesperson for the embassy, chose not to comment on what effect the evacuation has had.
“I have nothing to say other than the investigation is ongoing,” he told The Local.
The incident has been classified as a case of “spreading a poison or virus”.
The US embassy in Stockholm has around 170 employees, although it remains unclear how many were in the building on Wednesday at the time of the incident.