This is the first of a three-part series.
If a terrorist wants to get into the United States, all he has to do is fly a small plane across the border from Canada, below the radar.
Nobody will see him -- because nobody's watching.
With so much open space in places near the Canadian border like northern Minnesota, many people like to fly their small, private planes around the area. But others feel this poses a major security threat.
Frequent private flyers like Colin Peterson, who is also a congressman representing this area of Minnesota in the House of Representatives, don't like the idea of clipping the wings of pilots in the name of homeland security.
"If the terrorists get into the country somehow or another ... they're not going to come in by general aviation ... they're going to try to sneak in some other way," Peterson said.
Air travel by small planes means big money in northern Minnesota. These planes also bring in people who help fill resorts on both sides of the Canadian border, where tourism is big business.
But the federal government says it's conducting more inspections of people and cargo coming into the country since Sept. 11. When an aircraft lands in northern Minnesota from Canada or another foreign country, it's parked in a big yellow circle on the tarmac.
"We cannot touch that aircraft until Customs or Immigration has given us permission to service that aircraft," said Thor Einarson, manager of International Falls Airport.
And people take those rules seriously.
"If you don't clear Customs or Immigration going one way or the other ... you can wind up in jail or lose your plane ... fines or all of the above," said Tony Johnstone, a pilot from Kansas.
And you cannot just land anywhere.
"You have a certain number of airports, and they are limited across the northern border, that will allow pilots to land and clear Customs," said Phil Boyer of the Aircraft Owners & Pilots Association.
But there are other places in northern Minnesota where a plane can fly and not be seen by anyone. They can go forth across the border and the only security measures in place are the signs on the airfield.
The job of military men and women of the North American Air Defense (Norad) is to watch and protect America's skies. Norad is plugged into radar pictures all over the continent, but it admits the view near the Canadian border is, at best, incomplete.
"There are lots of places along the northern border that are not very ... interesting in terms of traffic," said Gen. Larry Arnold.
So how do Immigration and Customs officials make sure planes don't try to sneak past them?
"We don't. We don't," said Wally Schold, a U.S. Customs official. "There is no way we would know that."
What does exist, however, is what amounts to an honor system for small airplanes, and Peterson says that's just fine with him.
"These people aren't causing anybody any harm," he said.
Fox News went for a plane ride with Peterson to hear why he thinks the federal government ought to leave small airplanes alone.
"You would eliminate general aviation basically in this country," Peterson said. "You're going to eliminate guys like me being able to fly around like this."
Peterson believes that if radar ever covers the entire United States, small planes would get treated just like commercial airliners -- watched and directed by federal air traffic controllers. Filling in the radar gaps would also waste taxpayer money, he says.
"We're a lot less dangerous than a truck ... or even a van," Peterson said. "I'll fight this to the death."
And Peterson says Canada is a much bigger problem than the United States.
"The Canadians don't have the tough immigration laws coming into their county that we have," he said. "That would be a much better use of resources to figure out how to get them to tighten down on who they let into the … country."
Canada has what's called an "open door" immigration policy, where even people from terrorism hotbeds are let in, often without proper identification.
The Canadian Intelligence Service believes there's a connection between the country's immigration policy and the 50-some terrorist groups operating within their borders.
"I think it is fair to say that the anti-American ... the American-targeted-terrorist like the Islamist and others regard Canada as an ideally-situated place for some of their operations against the States," said David Harris of the Canadian Intelligence Service.
Peterson admits that if terrorists wanted to, they could fly a smaller plane into the United States undetected if they stay low enough flying in from Canada.
This infuriates Chicago Mayor Richard Daley, who calls small airplane travel the biggest security loophole in America.
"You can fly a single ... two-engine plane without authorization," Daley said. "You can fly anyplace in America ... except the White House."
And even small planes can do damage. That point was made earlier this year, when a suicidal teenager in Florida flew a small plane into a Tampa high-rise.
In the minds of many, the open land near the Canadian border is nearly impossible to defend.
"If the bad guy or bad guys want to get the job done -- trains, planes, automobile, a kayak, canoe, a pair of swim fins or a good pair of walking shoes -- they can get into the U.S. one way or another," Einarson said.
But that doesn't seem to bother some.
"I think out here we have a ... maybe it's a false security. I don't know," Customs official Schold said. "In this area ... we've always felt comfortable."
Tomorrow's story will include an account of the U.S. military who, on Sept. 11, tried to track down the hijacked jets without actually being able to see them.
Steve Brown is an author, radio broadcaster and seminary professor at Reformed Theological Seminary in Orlando, Florida.