America has tightened security along its border with Canada, but some security experts say the 4,000-mile northern passage remains full of holes — an inviting “open door” to terrorists seeking to enter the U.S.

Since 9/11, U.S. Customs and Border Protection has beefed up security to the north, tripling the number of border protection agents assigned there to more than 900.

But because agents can't be on patrol 24 hours a day, they say they rely heavily on the eyes and ears of people who live along remote areas of the border to report anything out of the ordinary.

"To us it's the most important, our citizen contact,” said Troy Bobbit, a Border Patrol supervisory agent. “Citizen reports are extremely important."

Bobbitt, who is based in International Falls, Minn., covers a section of the border separated entirely by lakes and rivers. Even though the water level is currently 3 to 4 feet above normal on Rainy River, which flows along the border, “you could use an inner-tube to float across,” he said.

“That river has parts that are shallow enough that in a dry year you could walk across it," he said.

Still, officials say it would be difficult to cross unnoticed, as agents share intelligence with Canadian authorities on a regular basis.

“I don't think it's easy for a known terrorist or criminal to enter Canada and approach our border and penetrate our border undetected," said Scott Baker, chief of U.S. Border Patrol for the Grand Forks Sector, which covers parts of North Dakota, Minnesota and Lake Superior.

But just last fall the U.S. Government Accountability Office released the unflattering results of its investigation on security at both the Canadian and Mexican borders.

"We went through official ports of entry where they had plenty of staffing but they couldn't recognize counterfeit documents," said Gregory Kutz of the GAO.

GAO investigators tried crossing the Canadian border illegally, carrying mock-radioactive materials, taping their attempt in the process.

Not once were they caught.

Click here to see video of the GAO officials crossing the border undetected.

Robert Leiken, director of the Immigration and National Security Program at the conservative Nixon Center, said such holes in the Canadian border make it "the real open door" to America, a conclusion he says he reached after studying the movements of known terrorists.

"They are heading almost universally to Canada if they don't come directly to the United States," Leiken said.

To confront the danger of illegal crossings, Customs and Border Protection have established the largest law enforcement air fleet in the country.

The North Dakota Air Guard Sector now has 23 pilots responsible for border protection, and unmanned aerial vehicles will soon be deployed to help monitor thousands of miles of territory separating the United States and Canada.

"It's not just one plane we're flying, it's not just one person involved, it's an integrated effort," said Mike Corcoran of the North Dakota Air Guard.

Security efforts along the border may be working. Since the increased presence of border protection agents, the number of arrests on the northern border has fallen by more than 40 percent to just over 6,000 last year.