The Delaware National Guard has increased security at an air base after different vehicles approached its gate this week, then left.

A spokesman at New Castle Air National Guard Base in northern Delaware says that in at least some of the incidents, an individual asked for directions. In at least one case, a vehicle turned around before reaching the main gate.

Col. Len Gratteri says the activity is not uncommon, but there were five such incidents. That frequency is what prompted base officials to increase security.

Gratteri says he does not believe the same vehicle or same individuals were involved each time. Investigators are reviewing security camera footage.

Vice President Joe Biden, who is from Delaware, has flown to the airport in the past on Air Force Two.