WASHINGTON - The breakup of an alleged terrorist bomb plot in Canada late last week has raised new fears that the next terrorist attack could come from the north.

The 4,000-mile border between the United States and Canada is twice as long as the one with Mexico, but it’s guarded by fewer than 1,000 Border Patrol agents - one-tenth of the force in the Southwest. Vast stretches of unpatrolled terrain offer potential terrorists an easily accessible gateway into the United States.

"Do the math," said T.J. Bonner, a San Diego agent who serves as the president of the National Border Patrol Council, a 10,500-member union that represents nonsupervisory Border Patrol agents. "We’re very vulnerable out there."

Although boosting border enforcement and national security have emerged as central elements of the immigration legislation that’s moving through Congress, attention has centered on the porous Southwest border, where more than 1 million illegal immigrants are arrested annually.

But some lawmakers are calling for a harder look at the northern border after Friday’s arrests of 12 men and five juveniles who allegedly tried to stockpile three tons of ammonium nitrate fertilizer to carry out terrorist bombing attacks in Canada. Timothy McVeigh used two tons of the highly destructive chemical substance in the 1995 attack that demolished the Oklahoma City federal building, killing 168 people.

Although the suspects appeared to be targeting sites in Canada, including the Parliament building in Ottawa, investigators fear they established a U.S. link with two terrorism suspects in Georgia who might have been plotting attacks in the United States. That remains unconfirmed.

The Canadian Security Intelligence Service, the government’s chief national-security agency, has estimated that as many as 50 terrorist organizations and 300 individual terrorism suspects have bases in Canada. Active groups, the agency said, include Shiite and Sunni Islamic extremists and all major Sikh terrorist organizations.

Even before the Sept. 11, 2001, terrorist attacks, the northern border had a reputation as a potential open door for extremists who’d settled in Canada to take advantage of its liberal immigration policies. One Canadian officer described the country as "Club Med for terrorists."