The police are now concerned that terrorists might dress up as police security, fire fighters, and even emergency medical technicians (EMTs) during Pope Francis' historic six-day visit to the United States beginning this Tuesday.

According to a document obtained by NBC News, the Pennsylvania State Police's Criminal Intelligence Center has warned its personnel and other law enforcement agencies that terrorists might be impersonating security and medical experts to enable them to enter secure areas and wreak havoc.

"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, which is titled "First Responder Impersonators: The New Terrorist Threat."

The document made no mention of the papal visit, but it was issued just days before Pope Francis' first visit to America. Citing reports from Homeland Security and the FBI, the bulletin narrated certain instances in the United States and other countries where suspects took advantage of wearing police uniforms to carry out an attack.

One such instance was back in March when authorities managed to foil the plans of National Guard trooper Hasan Edmonds, 22, and his cousin to kill dozens of people at a US military installation in Illinois using his uniforms. The two men reportedly wanted to fight for ISIS in the Middle East. They were caught and subsequently charged with conspiring to provide material support to a foreign terrorist organisation.

The bulletin warned that the US is particularly vulnerable because terrorists have the option to 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.

Police have already been advised to be careful of their belongings and act on any reports where first responder uniforms and credentials are stolen. They are also told to be on the lookout for "personnel with appropriate uniforms/vehicles in unapproved areas with no reasonable explanation."