Police are investigating a suspicious letter sent to the Bronx courthouse Tuesday, law-enforcement sources said.

A pasty white substance was found in an envelope along with a nasty letter to a judge, a courthouse source said.

“We think it’s nothing, but we’re having the police investigate just in case,” the source said, noting that the substance wasn’t powdery.

The NYPD’s Emergency Services Unit was called to the scene to examine a letter around 3:40 p.m.

The building was not evacuated.