(CNN) The U.S. Navy raised its threat level at a naval installation in Philadelphia Tuesday to its second-highest level after receiving a "potentially credible threat," a U.S. official said, but gave the all-clear about a half-hour later after police investigated.

The Navy was informed by the FBI of the threat to potentially take place at Naval Station Activity Philadelphia, which supports logistics, supplies and other offices, around 10:30 a.m., the official said. The threat level was raised to "Charlie," which indicates an imminent threat of terrorism. Under Charlie, non-essential workers would typically be sent home.

Authorities said both the Naval Yard and the Navy Base were evacuated. But Philadelphia police said an investigation turned up "no credible threat."

It was not immediately clear what triggered the alert.