On the eve of Pope Francis' historic first visit to the United States, law enforcement officials are concerned terrorists could disguise themselves as police officers, firefighters and emergency medical technicians to carry out attacks, according to a report from NBC News Monday. A memo titled "First Responder Impersonators: The New Terrorist Threat," from the Pennsylvania State Police's Criminal Intelligence Center and sent to law enforcement, warned that terrorists could falsely identify themselves as first responders to enter secure areas and carry out attacks.  

"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," the memo read, according to NBC News.

Pope, Police Police officers stood behind a cardboard cut-out of Pope Francis in Philadelphia, Pennsylvania, Sept. 16, 2015. Photo: Reuters

In particular, the memo from the Pennsylvania State Police's Criminal Intelligence Center warned that the United States is vulnerable to such an attack because terrorists can easily purchase first responder gear.

"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," the bulletin read, according to NBC. The memo advised that police look out for stolen uniforms or credentials, emergency vehicles in disrepair or loaded with a large amount of cargo and for uniformed personnel in unapproved areas without good reason. 

The bulletin, which did not specifically mention the papal visit but was released days before the planned event, cited a number of instances in which people wore or possessed police uniforms to carry out attacks. The Department of Justice said in March that authorities foiled a plan hatched by 22-year-old Army National Guard soldier Hasan Edmonds and his cousin to carry out an attack in Illinois using his uniforms. French authorities arrested in April 24-year-old Sid Ahmed Ghlam, foiling his alleged plan to carry out an attack on a church, and found weapons, bulletproof vests and police armbands. Both Edmonds and Ghlam were considered to be supporters of the so-called Islamic State. 

Authorities have said the security during the papal visit will be at levels typical for the presidential State of the Union address, reported CNN. Efforts will be especially intense during Francis' stop in Philadelphia, which is expected to host the largest public event on his U.S. tour. On Friday, U.S. Secretary of Homeland Security Jeh Johnson, referencing Philadelphia, said, "We know of no specific, credible threat directed at the pope's visit to this city," according to CNN. 

A 15-year-old Philadelphia boy was arrested in August for planning an attack on the pope, the teen reportedly inspired by the so-called Islamic State. The pope's stops in the U.S. have been designated "National Special Security Events," or events that are likely attractive targets for terrorist groups, according to a threat assessment by the FBI and the Department of Homeland Security, CNN reported.

The pope is scheduled to land in Washington, D.C., Tuesday, make a stop in New York City Thursday, and leave from Philadelphia Sunday after celebrating Mass.