WASHINGTON (AP) — Secretary of Defense Ash Carter has reminded the military service chiefs that Defense Department policy does allow some military personnel to be armed while working at recruiting centers and ROTC offices.

Carter directed the military branches Thursday to review security policies — including adding armed personnel — in the wake of the July 16 attacks at a Navy-Marine reserve center in Chattanooga that killed four Marines and a sailor.

Carter said in a memorandum released Thursday that the service secretaries should develop plans that include the option of armed personnel. Military personnel are generally prohibited from carrying firearms at recruitment centers and bases, but he reminded the chiefs that existing policy allows for armed personnel for security, law enforcement and counterintelligence duties.

Pentagon spokesman Capt. Jeff Davis said the memo is focused on facilities outside of military bases, such as recruiting stations and ROTC offices. He said there are about 7,000 such facilities throughout the United States.

"Looking at arming personnel doesn't mean that's what the services will ultimately decide," Davis said. "But it does tell them that they have, within DOD policy anyway, the existing authority to do that."

Davis acknowledged that the service chiefs will have to deal with potential conflicts at private buildings or on university campuses that restrict firearms.

"I'm sure that the service chiefs, as they look at this, will be well aware of the constraints that they face in doing this, and that will be part of their plans," Davis said.

Carter also says the secretaries should look at improving potential upgrades to equipment and buildings as well as procedures.

Carter's memo came after Kuwait-born Muhammad Youssef Abdulazeez, 24, of Hixson, Tennessee, unleashed a barrage of fire at a recruiting center in Chattanooga, then drove several miles away to a Navy and Marine reserve center, where he shot and killed the Marines, and wounded the sailor, who later died. Abdulazeez was shot to death by police.