The bases will maintain at force protection level "bravo" -- the third-highest threat level on a five tier Defense Department scale -- where they have been since May.
The command has "reviewed (its) force protection measures and provided specific direction regarding additional force protection measure to reserve centers, recruiting centers and ROTC units," Capt. Scott Miller, public affairs director for U.S. Northern Command, told CNN.
Miller said some of the measures will include increased vigilance and random security checks as well as some other, undisclosed measures.
The U.S. Northern Command oversees all U.S. military installations in the continental U.S. The security protocol affects 3,200 sites, including bases, National Guard facilities, recruiting stations and health clinics.
Since NORTHCOM was established in October 2002, the threat level has reached Bravo on four occasions: Feb. 9, 2003, amid concerns al Qaeda was planning attacks on American targets; Dec. 21, 2003, when officials were concerned about attacks during the holiday season; May 1, 2011, in the aftermath of the raid that killed Osama bin Laden; and the 10th anniversary of the Sept. 11 attacks.
It reached "Delta," its highest level, on Sept. 11, 2001, Pentagon officials told CNN at the time.