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With Pope Francis set to begin his first-ever U.S. visit Tuesday, a document obtained by NBC News shows law enforcement is worried terrorists may impersonate police officers, firefighters and EMTs to launch deadly attacks inside the U.S.

A memo distributed by the Pennsylvania State Police's Criminal Intelligence Center to law enforcement warns that imposters pretending to be first responders could use false identification to enter secure areas and wreak havoc before slipping away undetected.

“The impersonators’ main goals are to further their attack plan and do harm to unsuspecting citizens as well as members of the emergency services community,” said the bulletin, titled “First Responder Impersonators: The New Terrorist Threat.”

The document — which does not mention the papal visit but was issued just days before the event — cited instances at home and abroad where suspects donned or were in possession of police uniforms in a bid to carry out an attack. It was based in part on Homeland Security and FBI reports.

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In March, U.S. authorities said they derailed plans by Army National Guard soldier Hasan Edmonds, 22, and his cousin to kill dozens at a U.S. military installation in Illinois using his uniforms.

In this courtroom sketch, Jonas Edmonds, left, and Hasan Edmonds, right, stand in front of an FBI agent as they appear at a hearing at federal court in Chicago to face charges of conspiring with the Islamic State group. Tom Gianni / AP

The two men, who federal authorities said wanted to fight for ISIS in the Middle East, have been charged with conspiring to provide material support to a foreign terrorist organization.

In April, French authorities arrested ISIS sympathizer Sid Ahmed Ghlam, a 24-year-old Algerian national, and foiled his alleged plot to attack a church in Southern France. In addition to weapons, bullet-proof vests and notes detailing possible targets, Ghlam was also in possession of police armbands.

Belgian authorities discovered a weapons cache, police uniforms and fake IDs after a January firefight with terror suspects, and believe the suspects planned to launch an attack disguised as law enforcement.

The bulletin warned that the U.S. is vulnerable because would-be terrorists can shop at “numerous businesses that cater to the needs of first responder personnel."

"A wide variety of products such as clothing, weapons and tactical gear can be purchased on the Internet by any consumer, regardless of a confirmed affiliation to emergency services, government or law enforcement agency," it said.

The document advised police to be on the lookout for the theft of first responder uniforms and credentials, for emergency vehicles in poor repair or loaded beyond capacity, and for “personnel with appropriate uniforms/vehicles in unapproved areas with no reasonable explanation.”

The Pennsylvania Criminal Intelligence Center is a division of the Pennsylvania State Police’s Bureau of Criminal Investigation.

Pope Francis arrives at Maryland’s Andrews Air Force Base Tuesday night and will visit Washington, D.C., New York and Philadelphia before returning to Rome on Sunday, Sept. 27. Security preparations include the deployment of thousands of first responders in all three U.S. cities.