Eight people, including police officers, are in hospital after a mysterious incident at Sweden’s security service headquarters.
Around 500 people were evacuated after workers reported an unusual smell.
It was first reported as a suspected gas leak, but security services have since said no gas was detected inside or outside the building.
Police are looking into the cause of the alert and have launched a preliminary investigation.
Local media reports earlier suggested sensors on the building’s roof detected phosgene, but authorities have not confirmed this.
Phosgene is used to make plastics and pesticides, and was responsible for the vast majority of chemical deaths during World War One.
Emergency services launched a major operation after a call from the headquarters of Sapo, Sweden’s security service, in Solna, just outside Stockholm city centre, at 12:30 (11:30 GMT) on Friday.
The nearest exit of a nearby motorway was closed and barriers were set up hundreds of metres around the premises. The building was partially evacuated.
Six people were sent to hospital with reported breathing problems and two others admitted themselves. Among these were police officers who “noticed a smell when they arrived at the scene”, police said.
Images from the scene showed police officers wearing gas masks, and a helicopter circled the area. People in buildings nearby were told to shut their windows.
Schoolchildren were kept indoors, a teacher told broadcaster TV4. People who live in the sealed-off zone were refused access to their homes.
First responders ended their operation and removed barriers at about 16:30.
The incident came as Prime Minister Ulf Kristersson visited the capital of Hungary, the only Nato member that has not yet ratified Stockholm’s accession to the alliance.
The Hungarian parliament is expected to approve Sweden’s membership on Monday.