The threat level has been raised at the New Castle Air National Guard Base after five attempted security probes by unknown individuals this week, several federal and base sources tell NBC Philadelphia and NBC News.
One of the probes happened on Monday and four on Tuesday at the base along West Basin Road in New Castle, Delaware, counterterrorism and military sources said.
Base officials confirmed Wednesday morning that the base had increased security because of the security threat.
Vice President Joe Biden has used New Castle Air National Guard Base as it is close to his Delaware home. President Barack Obama has also visited the base twice.
The probes took place along the base's perimeter and the main gate by individuals inside vehicles who were described as "Mideastern" in appearance, sources said. A senior U.S. Defense official tells NBC News "no specific threat or individuals" have been identified.
A Delaware Air National Guard public information officer tells NBC Philadelphia's Tim Furlong the individuals drove up to several gates and asked for directions or said they were lost. In one case, the car drove away when a soldier approached the vehicle. They would not comment on the possible race of the individuals.
The threat level was raised two levels from normal to "THREATCON BRAVO" and the base has put in place a 100-percent identification check, locked gate policy and a search of all deliveries, according to sources. Base officials said it was done out of an abundance of caution.
Counterterrorism sources said the FBI, Joint Terrorism Task Force, Delaware State Police and U.S. Air Force are all investigating the incidents.
Delaware State Police tells NBC Philadelphia they have increased patrols around the base and other military facilities across the state at the request of the Delaware Air National Guard. The largest military installation in Delaware is Dover Air Force Base in Kent County.
This is a developing story. Check back for updates.