Several military families in Colorado and Wyoming reported being approached and intimidated by "Middle Eastern males" near their homes, and local law enforcement officials issued an internal "alert" seeking additional information.
The incidents began earlier this summer in a region where the Air Force maintains hundreds of nuclear missile silos.
The newly disclosed report from the Federal Bureau of Investigation's Denver office was dated July 2, several weeks before FBI Director James Comey visited the Colorado office and warned about the so-called Islamic State group's use of social media to radicalize young people living in the United States.
"This terrorism is in your pocket, on your hip, in your jacket pushing you all day long with slick propaganda," he said July 23.
According to the FBI report, two "Middle Eastern males" approached the wife of a military service member outside her Colorado home and told her that "she was the wife of a U.S. interrogator."
The two men left in a "dark colored, four-door sedan with two other Middle Eastern males in the vehicle," according to the FBI report. The woman contacted a local law enforcement agency, which in turn contacted the FBI.
"Similar incidents in Wyoming have been reported to the FBI throughout June 2015," the report said.
"On numerous occasions, family members of military personnel were confronted by Middle Eastern males in front of their homes. The males have attempted to obtain personal information about the military member and family members through intimidation. The family members have reported feeling scared," according to the report.
The FBI report was distributed internally to local law enforcement agencies in Colorado and Wyoming.
U.S. military officials declined to comment on the FBI report, but have repeatedly warned service members in recent months to take additional safety precautions.
In May, 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.
Gortney said the measure was not tied to a specific threat. It came shortly after the May 5 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, officials said.
The FBI report also came several weeks before the fatal shooting of five service members in Chattanooga, Tennessee. On July 16, Muhammad Youssef Abdulazeez opened fire on two U.S. military facilities, a recruiting center and a Navy reserve center. The motive for the shooting was unclear.