WASHINGTON (CNN) -- The Department of Defense raised the level of security conditions at U.S. military bases Thursday night, May 7, 2015, to "Bravo" because of the growing concern of terrorist threats in the United States.
But what does the Bravo level specifically mean?
The Force Protection Condition, or FPCON, is a five-tier threat level system that is overseen and decided by the Department of Defense. It has various levels that show security concern in the country. According to the list, the antiterrorism program "provides guidance and establishes standards for implementing the levels" in the Department of Defense. Local commanders decide which level is appropriate.
A U.S. Air Force document describes Bravo as when "an increased and more predictable terrorist threat activity exists." The level means there is information that suggests probable violence in the United States, and there must be extra precaution to deter terrorist planning.
The change could mean more checks of vehicles entering military bases and identity checks of all personnel.
The threat level systems were reassessed by the George W. Bush administration after the September 11, 2001 terrorist attacks.
The attacks were a "catalyst to review current Department of Defense policy guidance and resourcing for the conduct of force protection activities on all domestic military installations," according to a military research document.
Since the U.S. Northern Command 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.
Thursday's change comes after FBI Director James Comey made comments about Elton Simpson, one of the two Texas attackers of the Mohammed cartoon contest in Garland, Texas, who was shot and killed Sunday by a guard.
"I know there are other Elton Simpsons out there," Comey said, adding that the FBI has hundreds of investigations in the U.S. of possible extremists influenced by known ISIS recruiters.