RICHFORD, Vt. – In the frantic days immediately afterward, a Canadian connection seemed logical.

Even some Canadians saw their country as a haven not only for the persecuted, but for terrorists.

Alleged Sept. 11 mastermind Mohamed Atta and a partner began that day in Maine, where stretches of the 611-mile Canadian border have more moose than people. Early reports, quickly discounted, said they had come from Canada.

So far, authorities on both sides of the border say none of the 19 suicide hijackers had any Canadian connection.

But counterterrorism officials and academics are anything but reassured. They say the threat of Canada being used as a staging ground for terrorist attacks across the border is both real and serious.

“Nothing has changed. Hopefully it’s been disrupted and a lot of the people have been picked up,” said Harvey Kushner, a terrorism expert at New York’s Long Island University and security consultant to the U.S. government.

“But in terms of the support systems, the desire, the will, all of that still exists.”

The concerns are bolstered by more than two decades of history. The most recent item was in January, when a Canadian citizen was identified as one of five al-Qaida activists making suicide threats on videotape captured in Afghanistan. Tunisian-born Al Rauf bin Al Habib Bin Yousef al-Jiddi, 36, and Faker Boussora, another Tunisian-born Canadian described by U.S. officials as an associate of al-Jiddi, are believed to have left Canada in November.

Montreal, a scant 60 miles from the Vermont border at Highgate Springs, also is where Algerian Ahmed Ressam planned to bomb the Los Angeles airport to coincide with the millennium. An alert U.S. Border Patrol agent in Washington state nabbed Ressam in December 1999.

In 1998 a Lebanese man caught trying to enter the United States illegally at Champlain, N.Y., just west of Vermont, was linked to the Middle Eastern terrorist group Abu Nidal.

Fifteen years ago, three Lebanese men living in Canada were arrested in Richford after then-Police Chief Richard Jewett found a bomb one of the men had carried into the United States by walking across the border on a railroad track. The men were later connected to a Syrian terrorist organization.

Even further back, Kristina Berster, a suspected member of West Germany’s former Baader-Meinhoff gang, was captured entering Vermont from Canada in 1978. The gang, responsible for terrorist attacks in Europe in the 1970s, worked with Palestinian groups before eventually disbanding.

Since Sept. 11, U.S. agents have focused as never before on who and what is crossing into this country from Canada. They are peering into trunks, poking cargo, demanding identification and asking questions.

The Border Patrol has beefed up patrols and is doubling the number of agents on the northern border. While they wait for the new agents to arrive, Border Patrol officials are using officers temporarily assigned from the Mexican border.

The Border Patrol and the National Guard are both flying helicopter patrols and Guard soldiers are helping the U.S. Customs Service and Immigration and Naturalization Service search cargo and staff remote border crossings.

U.S. and Canadian authorities also are sharing intelligence and running joint operations along the border more than ever. The two countries had formed “integrated border enforcement teams” before Sept. 11, but the effort went into high gear when the terrorist attacks gave the work new meaning.

The cooperation extends from the highest levels, where the FBI and the Mounties share global intelligence, to street cops in Montreal and on the back roads of Vermont who are giving everyone and everything closer scrutiny.

“It just brought the best out of everybody. Everybody feels involved,” said Richard Huard, a Royal Canadian Mounted Police spokesman in Montreal.

On the other hand, the 3,987-mile border remains a warren of largely unpatrolled forests, back roads, prairie, farm fields and waterways.

“I think the border is probably impossible to secure,” concedes Rex Brynan, a political science professor at Montreal’s McGill University. “If the United States wants a secure border it is going to have to move to an island. That’s simply a reality of having a long border.”