The four-star commander who oversees U.S. military operations in North America ordered domestic military bases nationwide to increase their force protection measures amid heightened concern about terrorist threats.

Adm. Bill Gortney, commander of the Colorado-based U.S. Northern Command, raised military installations' force protection level to "Bravo" after months of maintaining it at "Alpha," the lowest level of security, a defense official said Friday.

"While our [force protection level] change is not tied to a specific, credible threat, recent events have led us to recognize the need to take prudent steps to ensure our [force protection] measures can be increased quickly and unpredictably at the macro level, just as they routinely vary at the micro level on individual installations," the defense official said.

The order comes after Sunday's shooting at a Prophet Muhammad cartoon contest in Garland, Texas. One of the shooters, Elton Simpson, 30, was a convert to Islam with a long history of extremism and had social media ties to militants linked to the Islamic State group in Iraq and Syria, law enforcement officials said.

Simpson and his housemate, Nadir Soofi, 34, were quickly shot and killed by a traffic officer after they managed to wound a security guard in the leg.

The director of the FBI warned Thursday about the threat of ISIS and its influence inside the U.S.

"I know there are other Elton Simpsons out there," FBI director James Comey told reporters. "We have a very hard task" in trying to stop individuals who might be influenced by ISIS and its propaganda via social media.

The heightened force protection levels will result in extra scrutiny at installation entry points and added security measures inside military bases.

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Raising force protection levels nationwide is intended as "a prudent measure to remind installation commanders at all levels within the USNORTHCOM area of responsibility to ensure increased vigilance and safeguarding of all DOD personnel, installations and facilities," the defense official said.

"This change, in addition to random drills or exercises, is a means to ensure that we effectively execute our force protection mission."