NEW BRUNSWICK —  The FBI is investigating what caused an employee to lose consciousness after handling an envelope in the Middlesex County Prosecutor’s Office mailroom on Thursday.

As a precaution, the building on Kirkpatrick Street in New Brunswick's Civic Square was evacuated about 2 p.m. and the air conditioning system was turned off to stop the potential spread of any airborne particles, according to a written statement from the city of New Brunswick.

The Middlesex County Hazardous Materials Unit gave the all-clear at 3:30 p.m. and workers were allowed to return to the building.

The condition of the employee and the addressee of the envelope was not revealed by the city.

The envelope, sent via the U.S. Postal Service, appeared to have nothing inside and was turned over to the FBI for investigation.

The building also houses the New Brunswick Police Department.

The FBI and the Prosecutor's Office on Friday morning did not respond to our request for more information.

Enter your number to get our free mobile app