Seven people were treated in hospital and Sweden's Security Service's (Sapo) office was evacuated Friday after a suspected gas leak there, authorities said, adding that the cause was unclear.
Images from the scene showed police officers wearing gas masks, alongside several ambulances and emergency vehicles as an area around the office was closed off.
"Around 1:00 pm today, there were indications that there was a dangerous substance at Sapo's offices," Patrik Soderberg, chief physician at local healthcare authority Region Stockholm, told AFP.
"So far, five people have been taken to hospital with respiratory symptoms and there are also two people that have sought treatment on their own for respiratory symptoms," Soderberg said.
Soderberg added that an investigation was ongoing but that they did not immediately have any information on what kind of substance they were dealing with.
Sapo spokeswoman Karin Lutz meanwhile told AFP that they had "taken measures and emergency services are on site" after receiving an alarm.
The Nordic country is on high alert as it is expecting to clear the final hurdle to its bid to join NATO on Monday, with the last holdout Hungary scheduled to vote on ratifying its membership.
Lutz added that the building had been "partly evacuated," but declined to give further details or comment on whether they suspected foul play.
Police said that an area of "a couple of hundred meters" around the building had been closed off after "a potential gas leak".
Police and emergency services were at the site and that some of those taken to hospital were police officers who had "smelled an odour when they arrived," police said in a statement.
Newspaper Aftonbladet said witnesses had reported smelling something that reminded them of paint, and that locals had been told to close their windows.
The intelligence service this week released a report warning of the threat posed by foreign state actors, pointing the finger at Russia, China and Iran.
Sapo said activities threatening Sweden's security were "conducted on a broad front, and foreign powers act offensively."