Officials say the Minneapolis office of the Federal Aviation Administration was evacuated after a letter containing a suspicious substance was found.

FAA spokeswoman Elizabeth Cory says four people were hospitalized as a precaution.

Cory says several agencies, including the Department of Homeland Security, were investigating what the substance was.

The office houses employees involved in aircraft certification, security, pilot licensing and regulatory compliance. Authorities say air traffic control functions are not located in the building and were not affected.

The Star Tribune reports Cory did not know how many workers were evacuated.

A spokesman for Allina Health EMS confirmed that four people were taken to Hennepin County Medical Center on Tuesday. He had no further information.