PHILADELPHIA (CBS) — Officials with the Philadelphia Fire Department responded to a hazmat situation in Center City Thursday morning.

The incident happened at 16th and Callowhill Streets at the field office of the Immigration and Naturalization Service.

According to investigators, a suspicious substance was found in an envelope at the building.

Hazmat crews, along with EMS and fire crews, responded to the scene.

After a thorough investigation, tests on the substance were negative.

Workers arriving for the day were kept outside the building as a precaution. The all-clear was given at about 8:30 a.m.