KANSAS CITY, Mo. -- Police and fire crews responded to a "credible threat" in the mailroom at the Internal Revenue Service office early Friday morning.

Officers were called about an unknown substance that appeared in the mailroom on 26th Street, police said.

The mail room was evacuated and shut down while hazmat crews evaluated the substance that was found in a bin of mail in the mailroom.

Hazmat officials told KCTV5's Eric Chaloux that initial reports came back "negative" on the yellow substance . Officials indicated they believed the substance to be harmless.

The FBI will now handle the case, police said. Hazmat crews left the scene around 8:45 a.m.