San Francisco schools have received a threat similar to ones received in Los Angeles, New York and Dallas this week.

Officials with the San Francisco Unified School District say Thursday that officials deemed the threat not credible. Classes are open Thursday.

Authorities say they inspected all schools and found nothing suspicious. However, they have increased the number of police officers on campuses. The district enrolls 57,000 students.

It is not immediately known when San Francisco schools received the threat, but other districts in Florida and Texas report receiving threats by email Wednesday night.

The developments come on the heels of Tuesday's districtwide closure in Los Angeles, sparked by an email threatening a large-scale attack. New York City schools received a similar threat, but officials there concluded it was a hoax.