Before the dust settled on the ruins of the World Trade Center, authorities began an intense, far-reaching investigation that seemed to center on one area — not New York, but the community just across the river: northern New Jersey.

Six of the suspected hijackers lived at one time in northern New Jersey. And of the 352 people being held for questioning in the worldwide post-attack sweeps, more than 20 lived in the state.

In the port city of Elizabeth, authorities pulled over a car that matched the description of one connected to the attacks. The three men inside, Ahmad Kilfat, Mohammad Mahmoud al Raqqad and Nicholas Makrakis, were carrying $11,000 in cash and one-way tickets to Syria.

In Bayonne, a quiet, heavily Polish city across from Staten Island, FBI agents hauled boxes and boxes of evidence out of an abandoned house belonging to an Egyptian man who left town abruptly about a month ago.

And Jersey City has been swarming with feds since the attack.

Two men from Jersey City, Mohammed Jaweed Azmath and Ayub Ali Khan, were picked up the day after the attacks on a flight from Newark to St. Louis that had had been diverted to San Antonio, Texas. Both men were carrying $5,000 in cash, hair dye and boxcutters. Both had lived in a non-descript red-brick apartment building in Journal Square, the heart of Jersey City, and a minute's walk from the mosque of Omar Abdel Rahman, the Muslim cleric now imprisoned as the mastermind behind the 1993 bombing of the World Trade Center — a plot Jersey City police still boast about helping to crack.

And late last week, The Jersey Journal reported that another Jersey City man, Egyptian-born Diaa Mohsen, was providing the FBI with names of Jersey City-area people who might be connected to the terrorists. Mohsen was already in custody in Palm Beach County, Fla., on charges that he and another Jersey City man, Rajaa Malik, originally from Pakistan, were trying to smuggle Stinger missiles, machine guns and grenade launchers out of the United States. The possible recipient of those missiles, feds say, was Usama bin Laden.

All this is enough to make any mayor cringe.

"As much as I love our city, we have a couple of bad guys," Jersey City Mayor Glenn Cunningham said in an interview, spreading his hands.

Northern Jersey — especially Hudson and Essex counties — was an obvious choice to begin hunting for collaborators in a foreign-hatched terrorism attack. The area is perhaps the most densely populated in the country and home to large immigrant communities from every part of the globe.

A visitor to Journal Square would find women in chadors and men in caftans, along with children speaking Cantonese, Polish or Urdu. Fidel Castro raised funds for his revolution in Union City, N.J., which has the largest Cuban-American population outside of Miami. Jersey City, which was the first American home to most of those who came through Ellis Island, still proudly calls itself "America's Golden Door."

"We’re probably the most diverse city in the country," Cunningham said. "Look around, you’ll notice people with saris, Sikh turbans, African garb. We see our diversity as an advantage for the city."

But that same point of pride may have turned out to be an advantage for terrorists working in the area.

The established Middle Eastern immigrant community on the west banks of the Hudson makes it easy for newcomers to fit in. Malik had owned a liquor store across the street from Jersey City's City Hall and was a member of the city zoning board. Khan and Azmath were newsvendors at Newark Penn Station, where people remembered them as likable and generous tippers.

"It's a good place to blend in," Jersey City Fire Chief Frederick Eggers said. "It's an open society. It's a good thing about the city they used against us."

It's also an ideal place to plan an attack on the city that towers over all of New Jersey.

The World Trade Center is 10 minutes away via PATH commuter trains, and the Holland and Lincoln tunnels closely link Hoboken, Weehawken and Jersey City to Manhattan. Many apartment buildings in Hudson County have a better view of the Twin Towers than most in Manhattan or Brooklyn.

The Muslim community in Northern Jersey — of which the vast majority of residents are, by all accounts, law-abiding citizens who detest violence as much as anyone — is feeling singled out these days. This in a community still smarting from its treatment after the 1993 bombing.

Muslims from Paterson to Jersey City are complaining about being questioned and having their homes searched. Authorities are paying visits to mosques and going door to door in apartment buildings with large numbers of Middle Eastern residents.  People of Middle Eastern appearance say they also have to put up with droves of reporters, nasty stares and the occasional brick through the window.

"You know, it's every day, man," one exasperated man said outside Azmat and Khan's apartment building.

But Jersey City Police Chief Peter Behrens said the intense law-enforcement scrutiny is a necessary part of the largest police investigation in American history.

"There are thousands of tips," he said. "And we have to filter through these."