Pope Francis' visit to Philadelphia next year will be the largest event in the city's modern history - and possibly its most daunting security challenge.

The city's police force has been preparing for months, spurred by the potential of the high-profile guest and more than one million people expected to flood the Benjamin Franklin Parkway to see him celebrate Mass on Sept. 27.

The added risks will likely equal one thing: added resources, according to Steven Bucci, a homeland security expert at the Heritage Foundation.

"There will be fighter planes on standby. There will be military assets," he said. "There will be . . . weapons of mass destruction teams, ones that can find and ones that can disarm and ones that can respond."

While the pope's visit was not confirmed until Monday, Philadelphia Police Commissioner Charles H. Ramsey said his office had been working under the assumption that Francis would attend the last two days of the World Meeting of Families, Sept. 22 to 27, ever since it was announced last year that the event would be here.

The city's police force will be joined by a half-dozen other agencies during the week of events, including the Pennsylvania State Police, the Secret Service, the FBI, Vatican City police, Italian police, and the Swiss Guard, the pope's primary security detail.

A spokeswoman from the World Meeting of Families said it was premature to estimate the cost for security - which the group will cover.

Ramsey declined to say how many officers would be on duty for the visit but said his department would limiting time off. In addition to having a significant police presence, the department will use cameras and other technology to help maintain surveillance of the Parkway area.

"We want this to be something people come to and they don't really see us," he said. "We want to be as invisible as we can but still provide security."

Ramsey said the FBI will keep his staff apprised of any potential security threats. He said he knew of none now.

Bucci said threats are sure to arise but should not dissuade those who want to attend. He said the event will likely receive the same designation by the federal government as gatherings such as the Super Bowl or the State of the Union address, meaning an arsenal of resources.

"The city of Philadelphia will have more folks there to help them than they might even want," he said.

Mayor Nutter called the undertaking "significant from a security standpoint," but said the city is seasoned at hosting dignitaries and large events.

"This is slightly bigger, with a few more zeros on the end in terms of the number of folks coming," Nutter said. "But we'll be ready."

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