Washington (CNN) Security conditions at U.S. military bases have been increased over growing concerns about terror threats, officials said Friday.

A U.S. official confirmed to CNN that U.S. military bases are now at "Force Protection Bravo," which is defined by the Pentagon as an "increased and predictable threat of terrorism." It is the third-highest threat level on a five-tier scale used by the Department of Defense.

U.S. military officials added Friday that the announcement, which comes in the aftermath of the shooting at a Texas cartoon contest featuring drawings of the Prophet Mohammed, was not the result of a specific threat but because the military had become concerned about several recent incidents.

The military became alarmed when one of the jihadists linked to the Garland attack tweeted the name and address of a U.S. military officer connected to the military's Syrian rebel training program, a U.S. military official told CNN.

The tweet, first published on an account connected to British-born jihadist Junaid Hussain, was sent out a few days before the attack on the cartoon exhibit and appeared to encourage an attack on the address.

Photos: Two shot and killed at 'Muhammad Art, Cartoon Contest' in Texas Photos: Two shot and killed at 'Muhammad Art, Cartoon Contest' in Texas A member of the Garland Police Department keeps members of the audience inside the Curtis Culwell Center in Garland, Texas, after reports of shots fired outside. Two men were shot outside the event, called the Muhammad Art Exhibit and Cartoon Contest. The exhibit was sponsored by the American Freedom Defense Initiative. Hide Caption 1 of 4 Photos: Two shot and killed at 'Muhammad Art, Cartoon Contest' in Texas People are sequestered by members of the Garland Police Department inside the Curtis Culwell Center. Hide Caption 2 of 4 Photos: Two shot and killed at 'Muhammad Art, Cartoon Contest' in Texas Members of the Garland Police Department stand inside the Curtis Culwell Center. Hide Caption 3 of 4 Photos: Two shot and killed at 'Muhammad Art, Cartoon Contest' in Texas Members of the Garland Police Department stand at an entrance of the Curtis Culwell Center. Hide Caption 4 of 4

Hussain is the same jihadist who is also believed, according to U.S. law enforcement officials, to have been messaging with Elton Simpson, one of the two attackers of the Texas event, and had been urging Simpson to take action.

That followed the publishing several weeks ago on ISIS-connected accounts of the name and addresses of about 100 military members.

On Thursday, FBI Director James Comey told reporters that there are thousands of ISIS, also known as ISIL, followers online in the U.S.

"We have a general concern, obviously, that ISIL is focusing on the uniformed military and law enforcement," Comey told reporters Thursday.

The order to upgrade the threat level was signed by Admiral William Gortney, head of the U.S. Northern Command, which oversees all U.S. military installations in the continental U.S. The security order affects 3,200 sites, including bases, National Guard facilities, recruiting stations and health clinics, a Pentagon official said.

"We have the same concern about the potential threat posed by violent homegrown extremists," said Captain Jeff Davis, spokesman for the U.S. Northern Command, or NORTHCOM.

Davis declined to specify the new security measures.

But the change in threat level status could mean more checks of vehicles entering bases, and more thorough identity checks of all personnel. Davis emphasized that "this is the new normal, that we are going to have increased vigilance and force protection. We seek to be unpredictable."

A U.S. military official said the order to raise the force protection level to Bravo also applies to all National Guard installations, recruiting stations, and ROTC detachments, though practically speaking, the official acknowledges it will be difficult for the ROTC detachments to do much more than security awareness.

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In addition, security was raised recently at Wright-Patterson Air Force Base near Dayton, Ohio, in response to a perceived threat to the base security, another U.S. military official said. The threat was never deemed credible, but it came after another security concern at a base in Delaware used by Vice President Joe Biden when he flies home.

On Friday, Wright-Patterson announced that the Air Force museum, which is part of the base, was canceling a planned Friday night concert and was stopping tours that were regularly offered until further notice. The base said this was "due to elevated security measures."

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.