A joint-intelligence bulletin, released to law enforcement agencies days ago by the FBI, warns that terrorists could use marked police, firefighting or other emergency units, impersonating officers and officials, to launch attacks during the three-city papal visit this week by Pope Francis.

The bulletin came to light Monday when NBC News obtained a memo distributed by the Pennsylvania State Police Criminal Intelligence Center, NYPD Deputy Commissioner for Counterterrorism and Intelligence John J. Miller confirmed at a Tuesday morning television appearance with NYPD Commissioner William J. Bratton on "CBS This Morning."

Bratton and Miller said there were no specific threats and when asked if officials expected the pope would have guardian angels looking after him during his visit to New York City on Thursday and Friday, Bratton joked, "He's going to have 6,000 guardian angels." Meaning the NYPD.

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But Miller, referencing the bulletin, which NBC said Monday was titled "First Responder Impersonators: The New Terrorist Threat" and does not specifically mention the papal visit, said it is a warning to law enforcement agencies that terrorists could impersonate officers and other responders to carry out attacks -- and that the NYPD, Secret Service, FBI and other agencies will be acting accordingly during the visit by Pope Francis.

"Everybody's going to be challenged," Miller said Tuesday, stating that every officer, every official, in every unit, including marked units, including everyone in uniform, will be questioned at every checkpoint. And, Miller said of officers and officials when questioned: "You'd better know what you're talking about."

The pope begins his visit to the United States on Tuesday with a trip to Washington, D.C. He also will visit Philadelphia.

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Miller said that on Monday officials distributed a "joint-intelligence product" in conjunction with law enforcement agencies in the three cities that reviewed the potential threat picture -- and reviewed past attacks on pontiffs during papal visits worldwide.

"We wanted to remind everyone the ways these things have happened in the past," Miller said. Past attacks include the shooting of Pope John Paul II in St. Peter's at the Vatican City in May 1981 and an assault by a woman who rushed Pope Benedict XVI during Christmas Eve midnight Mass inside St. Peter's in December 2009 -- knocking the pope and French Cardinal Roger Etchegaray to the ground, breaking the cardinal's leg and hip.

Miller said that the most difficult aspect of battling terrorist attacks in the Internet age is that recruitment and planning often takes place "on a glowing laptop in a darkened bedroom" and that such scenarios are "very hard to penetrate."

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On Monday, Miller said another hurdle facing authorities this week is that not only will the NYPD be tasked with guarding the security of the pope during his visit to New York City, but that the city is hosting the United Nations General Assembly, with 170 world leaders descending on New York -- not to mention an expected visit by President Barack Obama. These factors make this operation "the single-largest and most-complicated security undertaking" in the history of the NYPD, Miller said Monday.

That said, Bratton said Tuesday that police and security agencies have established a "pretty tightly coordinated" game plan to protect Pope Francis and the hundreds of thousands who will come to see him here in New York City.

"What's a celebratory event for everybody is a security event for us," he said.

The FBI, the Secret Service and the NYPD will strike a "balance" in protecting Pope Francis during his visit to New York City on Thursday and Friday while also respecting his tendency to spontaneously dip into the crowd or allow people to come up to his vehicle, Mayor Bill de Blasio said Tuesday morning.

"We're going to have extensive, careful security," de Blasio told CNN's "New Day," "but we have to recognize that if His Holiness wants to stop and greet people, that it's our job to protect him no matter what he does."

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The mayor added of the pontiff: "He will make that choice, so it's a balance that we're striking. But we feel very confident. We have the personnel, we have the equipment, we have the training. The pieces are in place."

Francis will have processions along Fifth Avenue and through Central Park where the ticketed public may view him, but the mayor and other city leaders have said security will be very tight. The pope's schedule includes evening prayers at St. Patrick's Cathedral, a visit to an East Harlem Catholic school, a meeting at the Sept. 11 Memorial and Museum, remarks at the United Nations General Assembly and a Mass at Madison Square Garden.

Everywhere the pope goes, he and the public can expect to be safe because the NYPD has anti-terrorism experience, de Blasio said.

"It's fair to say security here is different than any other place in the world," the Democrat said. "In New York City, we take very seriously, very soberly the fact that we have experienced terrorism. We're the No. 1 terror target in the world, so we have a very strong anti-terror apparatus within the NYPD."

Officials last week stressed that there is no known credible threat involving Pope Francis or the UN meeting.

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Rep. Peter King (R-Seaford) told Newsday, however, that the federal joint terrorism task force investigated a man at a location outside New York City who had made threatening remarks about the pope.

The threats weren't connected to any terrorism but were more along the lines of venting anger, said King, who declined to elaborate. FBI officials last week also declined to discuss the matter.

While three previous popes have visited the city since 1965, when Pope Paul VI became the first pontiff to do so, Francis' visit represent unique challenges because of his freewheeling nature and desire to be close to the people.

He will be bringing his own vehicle from Rome, reportedly a modified Jeep Wrangler, for the ride through Central Park. Francis is likely to get close to people, making police and federal officials anxious.

"In the course of his time here, he will be exposed to hundreds of thousands of people," Bratton said.

While Bratton joked last week about the pope having to pay his fare if he rides the subway, a church official in New York said privately that it is highly unlikely that Francis would attempt do so.

The NYPD expects to have about 6,000 officers assigned to the papal visit and UN General Assembly events. Some of that deployment will be covered by overtime payments, about which Bratton would only say "the cost will be what it is."

Police are also assigning a lot of hardware and vehicles to cover the events. Scores of emergency service vehicles are being marshaled. They contain an array of chemical, biological and radiation detectors. Individual cops will also be wearing smaller detectors with their uniforms.

Tactical officers assigned to the newly formed special response group, a unit of about 450 made possible by the addition of more than 1,200 new cops, will have access to heavy weapons and combat gear. Police aviation and harbor units also will be deployed, as will canine and mounted units.

Each of the airborne and harbor units also will carry radiation detectors to provide a wider sweep for monitoring. The city Office of Emergency Management will be open round-the-clock for Francis' visit to coordinate city responses to such emergencies as power outages and water main breaks, said OEM chief Joseph Esposito.

In fact, a power outage was part of one recent exercise conducted at police headquarters to test the response of police and emergency services during the week of heightened security.

With Anthony M. DeStefano