U.S. Navy Yard in Philadelphia.

 U.S. Navy Yard in Philadelphia. Google

The U.S. Navy Yard in Philadelphia got an "all clear" Tuesday afternoon after a threat forced some evacuations and triggered heightened security measures, officials told NBC News.

The threat, the nature of which was not immediately known, was eventually deemed not imminent.

Some buildings were at least temporarily evacuated. The Navy Yard had said its non-essential personnel had been sent home for the day, but the yard remained open.