Eight hospitalized and hundreds evacuated in mysterious incident at Swedish security services HQ

Initial reports from the BBC suggested a possible gas leak. However, upon reaching the site, the sensors did not show any deviations from the norm inside or outside of the building.

Earlier reports from local media suggest that sensors on the building's roof detected phosgene, but officials have not yet confirmed this. Phosgene, a chemical used in the production of plastics and pesticides, accounts for the vast majority of deaths during World War I.

A nearby highway exit was closed off by authorities, who also established a security perimeter around the site. Images of the scene show police officers donning gas masks and a helicopter surveying the area. Nearby residents were advised to keep their windows closed.

"Six people were sent to the hospital for examination due to reported respiratory issues. Additionally, two first-responding police officers have undergone examinations," the police reported.

Residents in the contained area were prohibited from returning home until the operation finished around 19:30 ET on Friday. The BBC mentioned the incident's timing, which overlapped with the visit by Sweden's Prime Minister, Ulf Kristersson, to Hungary - the only NATO country yet to approve Stockholm's alliance membership.

The Hungarian parliament is expected to approve Sweden's membership on Monday.