The 4,000-mile northern border the United States shares with Canada is the world's largest between two countries.
Only 1,000 Border Patrol agents patrol the area, though. That's just one-tenth of the force arrayed along the Mexican border.
"There's 30 roads between Minnesota and Canada that don't even have checkpoints," said Randall Larsen, a homeland security analyst. "You can just drive across them."
Some lawmakers say this weekend's arrest of 17 men suspected of a terror plot in Canada should serve as a wake-up call. The suspects had connections to two Georgia men arrested for suspected terrorist activities, ABC News has reported.
"Obviously, there are great needs in the South, and we don't dispute that," said Sen. Carl Levin, D-Mich. "But the Northern border is an area where we have tried desperately to get greater assistance from the federal government."
Starting in 2008, a new law will require Canadians to show passports or special pass cards when they cross the U.S. border. Governors in all six New England states oppose that law and worry that it will keep out businesses while monitoring potential terrorists.
Canada is America's No. 1 trading partner. There are many Americans and Canadians who cross the Northern border daily to go to school, to work, and to buy groceries. Some leaders in both the Democratic and Republican parties argue the bottlenecking that would result from heightened security would hurt trade and tourism.
U.S. Border Patrol Chief David Aguilar told The Associated Press that the Northern Border Patrol agents -- especially those near Ontario -- were on high alert because of the recent arrests. The agents will work extra hours, and some will move closer to the area where the suspects were arrested, Aguilar told AP.
In 1999, terrorists tried to breach the border. U.S. border agents foiled the so-called millennium plot when they stopped a Canadian man heading for the Los Angeles airport in a car loaded with explosives.
A new State Department report expresses growing concern that weak immigration laws in Canada make it a "safe haven" for terrorists. Experts say guarding every mile of the border with its dense forest and tall mountains would be impossible.
"It would take more than the gross national product of the United States and Canada to secure that border, and I think we really have to ask ourselves why we would want to," said Richard Clarke, a former U.S. counterterrorism czar and an ABC News consultant.
ABC News' Nancy Weiner originally reported this story.