The Marine Corps closed recruiting stations within 40 miles of the Thursday shooting, and has told workers there not to wear military uniforms, Pentagon spokesman Peter Cook said.
Cook said other military branches are also moving to increase security, at least temporarily. Navy recruiting stations will increase their coordination with law enforcement in the southeastern U.S., and the Army has bulked up security at certain recruiting stations.
"The Department of Defense continues to gather information on the circumstances surrounding the tragedy in Tennessee, including the specific security measures in place at the two facilities," Cook said.
Secretary of Defense Ash Carter has asked for more recommendations on how to better protect service members by the end of the week, Cook added.
Also in response to the shootings, the governors of Florida, Texas, Oklahoma and Indiana have taken steps to increase security of National Guard recruiters and military facilities in their states. States control their National Guard units, so governors can make decisions about Guard actions, whereas the President is commander in chief of the nation's military branches.
Under Florida Gov. Rick Scott's order, National Guard members at six state recruitment centers will be relocated to armories until security is improved. In addition, law enforcement agencies will be asked to conduct regular security checks and qualified Guard members will be adequately armed, according to a statement from the governor's office.
"We're going to do everything we can to make sure all of our Guardsmen are safe," Scott told CNN. "We've got to understand that we have people in our country that want to harm our military."
Texas Gov. Greg Abbott's order will arm National Guard personnel at military facilities throughout the state.
"Arming the National Guard at these bases will not only serve as a deterrent to anyone wishing to do harm to our service men and women, but will enable them to protect those living and working on the base," Abbott said in a statement.
Oklahoma Gov. Mary Fallin authorized the arming of certain full-time personnel in military installations throughout the state. "It is painful enough when we lose members of our armed forces when they are sent in harm's way, but it is unfathomable that they should be vulnerable for attack in our own communities," she said in a statement.
Indiana Gov. Mike Pence issued an order to enhance security at all National Guard facilities across the state, including recruiting storefronts.
Mohammad Youssuf Abdulazeez killed four Marines and wounded three others -- including a Navy petty officer who died early Saturday -- in what authorities believe may have been an act of domestic terrorism. Police shot and killed Abdulazeez.