(CBS) To combat drug smuggling and illegal immigration, the federal government is fortifying the Mexican border -- adding thousands of new patrol agents, surveillance cameras, sensors and unmanned aerial drones.

In contrast, the Canadian border is a 5,000 mile expanse, which a new government investigation warns may be a welcome mat for terrorists with weapons of mass destruction.

A report by the Government Accountability Office obtained by CBS News concludes: "A determined cross-border violator would likely be able to bring radioactive materials ... undetected into the United States..."

Over eight months, government investigators crossed the northern border unchallenged at least six times, carrying a small duffel bag meant to represent a "dirty bomb." The teams did not carry any real explosives or radioactive material, but carried simulated cargo, that, upon further inspection, would have seemed suspiciously like the real thing.

Border roads were "unmanned and unmonitored" with barriers "that could be driven around."

In one case, a citizen alerted authorities, but "the U.S. Border Patrol was not able to locate the investigators with the duffel bag."

The May 2007 findings also point to a great disparity in manpower, with 11,986 Border Patrol agents along the Southern boundary and just 972 patrolling the much longer Northern front.

Security officials argue the Southern border poses the greatest risk.

"When you take a look at the challenges with the economic migrants coming across, as well as the criminal migrants coming across, as well as the 2 million pounds of drugs that cross into the Southern tier into the United States, that's a huge threat that needs to be addressed,” says Jay Ahern of U.S. Customs and Border Protection.

But the Northern border has been tested before. So-called millennium bomber Ahmed Ressam was caught with a car full of bomb parts while crossing near Seattle.

"Take Ahmed Ressam and put him somewhere over North Dakota or Minnesota, at a sparsely populated border crossing, and he would have been inside the country pretty quickly,” says CBS News Homeland Security consultant Paul Kurtz.

Investigators credit the government for greatly improving border defenses since 9/11, but say it's clear more manpower and resources are badly needed to protect against a threat from the North.

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