New York City and Port Authority police formed a blue wall around lower Manhattan Sunday morning as ceremonies marking the 10th anniversary of the Sept. 11 terrorist attacks began amid concern over a threat of a new attack using a car or truck bomb.

Service on The Port Authority's PATH train was suspended between Exchange Place and the World Trade Center until 11 a.m., and the Metropolitan Transit Authority had the R train skip the Courtland Street stop until early afternoon.

Families of the Sept. 11 victims began arriving at security checkpoints at West Broadway and Warren Street near dawn and showed invitation letters to security staff wearing orange ball caps.

Security was also ramped up at Penn Station this morning. Soldiers with automatic weapons lined the terminal and city police set up several areas where they checked bags. State police and a number of firefighters wearing their dress uniforms were also at the station.

Richard Meir, a tourist from Germany, left Penn Station for a little sightseeing in Harlem. He noticed the large police presence but said he wasn't surprised, adding "you have to be well prepared."

NYPD also closed streets in much of the financial district, creating a frozen zone, before the 9/11 ceremony began.

Since late Wednesday, counterterrorism officials have been chasing a tip that al-Qaida may have sent up to three men to the United States on a mission to detonate a car bomb in New York City or Washington, D.C., on the 10th anniversary of the attacks.

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Should that mission prove impossible, according to the tip, the attackers have been told to simply cause as much destruction as they can, one federal official told The Associated Press.

At least two of those men could be U.S. citizens or have U.S. papers, The Associated Press reported.

Intelligence officials say they have no evidence there is anyone inside the United States tied to the plot and there has been no corroboration of the tip.

NYPD officials have said security, including checkpoints to inspect trucks and vans around Manhattan, would stay in place until at least Monday.

But even as extra police and national guard soldiers manned Manhattan's train stations and rail centers, federal government officials acknowledged that they have found no evidence that al-Qaida has sneaked any terrorists into the country for an attack on or near the anniversary.

Tough security was on the minds of camera-toting tourists making pilgrimages to Ground Zero for the anniversary. Rumy Yamada of Nagoya, Japan, used to live in New York City but moved back to Asia just before the terrorist attacks a decade ago.

This morning, she walked south on Centre Street, about a half-hour before the 8:46 a.m. commemoration, hoping to see the memorials. She said she has come to the city almost every year in honor of 9/11.

"Each year the security has become tighter," she said, noting that this is perhaps as heavy as she's seen it. "And last year it was like a circus."

Elsewhere in lower Manhattan, the wheels of the criminal justice machinery clanked along. Prisoners were booked into The Tombs jail. Court officials made preparations for the 1 p.m. arraignments.

Everywhere, though, there were more police, and memories of the slain weren't forgotten.

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"We lost three guys in the attacks" one court officer said.

Closed streets included:

* Northbound West Street between Battery Place and Murray Streets

will be closed to traffic from Friday, Sept. 9, 11:59 p.m. through

Monday, Sept. 12, 5 a.m.

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* Southbound West Street between Battery Place and Chambers Street

will be closed to traffic from Sunday, Sept. 11, 5 a.m. through

Monday, Sept. 12, 5 a.m.

* Church Street/Trinity Place between Rector and Barclay Streets

will be closed Sunday, Sept. 11, 5 a.m. through 5 p.m.

* Broadway will remain open to vehicles.

The following streets were closed to traffic and parking from

through 8 p.m., and pedestrian access was limited:

* Warren Street between West Street and West Broadway

* Murray Street between West and Church Streets

* Barclay Street between West and Church Streets

* Park Place between Greenwich Street and Broadway

* Greenwich Street between Warren and Vesey Streets

* West Broadway between Warren and Vesey Streets

* Vesey Street between North End Avenue and West Street

* Vesey, Fulton, Dey and Cortlandt Streets between Church Street

and Broadway

* Liberty Street between South End Avenue and West Street, and

between Greenwich Street and Broadway

* Albany Street between South End Avenue and Greenwich Street

* Cedar Street between Greenwich Street and Broadway

* Thames Street between Greenwich and Broadway

* Carlisle Street between West and Greenwich Streets

* South End Avenue between Liberty and Thames Street

* Washington Street between Albany and Rector Streets

* Greenwich Street between Liberty and Rector Streets

With Sophia Chang, Matthew Chayes and Robert Lewis