STOCKHOLM, March 29 (Xinhua) -- The white powder contained in letters sent to many Swedish authorities on Wednesday has been deemed "harmless", local media have reported.

At least three people were seen by medical personnel after 18 of Sweden's county administrative boards received letters containing a powdery white substance on Wednesday, Dagens Nyheter (DN) newspaper reported.

Analyses from several of the sites later showed that the substance was harmless, the report said.

Many buildings were evacuated following the discovery of the letters, and local authorities were unreachable by phone, as the main switchboard for the vast majority of the county administrative boards is centralized to one of those affected.

At least three employees were seen by medical personnel, while others underwent decontamination procedures and were placed in isolation as a precautionary measure.

Several letters also contained verbal threats, DN reported.

A spokesperson for the Swedish Security Service told DN that they were aware of the letters, but declined to comment on their involvement in the investigation.

No arrests have yet been made in the case.