DETROIT — The FBI says a threat that prompted Detroit’s public schools to step up security isn’t credible.

The Detroit Public Schools Community District says classes weren’t being interrupted Wednesday, but security was up as police investigated.

The FBI says in a statement the threat was received by fax Tuesday at a Detroit school and it determined there’s no credibility to the threat. The FBI is coordinating with local, state and federal law enforcement agencies to investigate.

Officer Shanelle Williams, a spokeswoman for Detroit police, told The Detroit News that her agency didn’t believe the threat was credible.

The school district’s police department didn’t release details about the threat, but said it was from an unidentified source. The district says the “safety of our students and staff is our highest priority.”