Federal officials and organizers said Monday that they are discussing the possible construction of a fence as high as 8 feet around parts of Center City as security for Pope Francis' visit in September, but that talks are still preliminary.

A source involved in the event planning said portions of Center City would be surrounded by fencing, but that the footprint of the security perimeter is being worked out and is largely contingent on the pope's Philadelphia itinerary, which could change in the next three months.

The U.S. Secret Service, which would have the fencing constructed, said the barriers are not definite.

"We are still almost three months out, and these things change almost daily," said Robert Hoback, a spokesman for the Secret Service. "It's very fluid, so I cannot say there will or will not be security fencing."

With fewer than 90 days until the pope arrives as the highlight of the World Meeting of Families, logistics for how the city will safeguard and manage him amid crowds estimated at 1.5 million or more are beginning to take shape. The Vatican released the pope's itinerary last week, adding a visit to Curran-Fromhold Correctional Facility and a speech at Independence Mall to plans for two major events on the Benjamin Franklin Parkway.

Hoback said specific information about street closures and security perimeters would not be released until three weeks before the event.

"There are many options, possibilities, still being discussed; i.e., where exactly the secure perimeter will be, what road closures will be in effect, where the magnetometer checkpoints will be located, and what folks will be allowed to bring into the venues," he said. "It's way too early to speculate."

People familiar with the planning said some things are certain, however.

Parked vehicles will not be allowed within the secure (likely fenced-in) perimeter, with some exceptions for emergency and other essential vehicles, which will be swept by the Secret Service. Certain streets will be closed to cars and buses. Those closures are still being determined, but early plans are from the Delaware River to the Schuylkill as far north as Girard Avenue and as far south as South Street, beginning hours ahead of the pope's Sept. 26 arrival.

The Secret Service has erected 8-foot, anti-scaling fencing to secure large areas, such as during the presidential inauguration in 2009.

Last month, the Vatican Embassy in Washington withdrew a request to erect a fence around its campus during the pope's visit to the capital, where Francis will go to the White House and address a joint session of Congress. The proposal for the 6-foot security fence - with wrought-iron fronting and chain-link sections - was suggested by the Secret Service in April, according to news reports. After some pushback from city officials and residents, the Vatican withdrew its application to the district zoning board.

The pope is expected to arrive in Philadelphia from New York City on Saturday morning at Atlantic Aviation adjacent to Philadelphia International Airport. He will say Mass for 1,600 people at a ticketed appearance at the Cathedral Basilica of SS. Peter and Paul and will make visit St. Charles Borromeo Seminary in Wynnewood.

The heaviest security begins that afternoon, when Francis is to speak in front of Independence Hall, with his themes being religious liberty and immigration.

After the speech, Francis will participate in the Saturday evening "Festival of Families" faith and music event on the Parkway, which will feature appearances by the Italian tenor Andrea Bocelli, the Colombian superstar Juanes, and the Philadelphia Orchestra. Sources said that for both the Saturday festival and the pope's 4 p.m. Sunday Mass, organizers are trying to spread the huge expected crowd away from the Parkway and over as much of the city as possible. To help with that, Jumbotrons will be erected for viewing, possibly at Penn's Landing or Independence Mall.

SEPTA has already announced that train, subway, and trolley routes will be severely altered that weekend. Most will pick up passengers from only some outlying stations and will run directly into the city. Only 31 of SEPTA's 282 subway-El, Regional Rail, and subway-surface stations will operate.

Mayor Nutter has warned that people need to be prepared to walk long distances. Hoback, of the Secret Service, said wheelchairs will be allowed into the event, though security screenings for those using them are still being worked out.

The city's Office of Emergency Management has created a grid system to help emergency responders and the public better figure out where they are on the Parkway. Alphanumeric signs have been posted along the Parkway, beginning at the Art Museum with the letter A.

The country flags on the Parkway rotate, making them difficult for first responders to rely on as location points, said Samantha Phillips, director of emergency management. "Plus, I don't know how many people know the countries by flag," she said. Phillips said the main purpose of the grid is to help the public. Many families are expected, she said, and the easier it is to find lost family or friends, the better.

Nutter joined Archbishop Charles J. Chaput, other Philadelphia Archdiocese officials, and World Meeting planners on a recent planning trip to Rome, largely dealing with security and logistics.

While there, Chief Inspector Joe Sullivan, who heads the city's Homeland Security and Counter Terrorism unit, said his advice for the throngs of people coming to the Parkway is patience.

"They need to come by way of public transportation, they need to leave very early - they need to realize they're going to have to wait in long lines to get into the venue, and then, once inside, be prepared to stand for long periods of time," he said.

"We want their expectations to be: It's a walking event, but it's going to be a great event, a magnificent event, and we're extremely excited about it here in Philadelphia."

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