For years America's northern and southern borders — 5,938 miles of dense forest and open land — were fair game for drug dealers and illegal immigrants looking to sneak into the U.S. That was before September 11, 2001.

After the 9/11 terror attacks, securing America's borders became a national priority, says Jay Ahern, deputy commissioner of U.S. Customs and Border Protection.

"There was an absolute shift in thinking about the border, from drug interdiction, alien interdiction, criminal interdiction at our borders to one of national security," Ahern said.

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In the last seven years there has been a huge expenditure of money and resources to prevent an attack on America from the north or south.

The U.S. Department of Homeland Security was formed and, under it, Border Patrol was flooded with new resources.

By the end of this year, the number of agents patrolling the border is expected to be 18,000, double what it was eight years ago. About 338 miles of fence have been built in the south, and more is planned.

The Border Patrol has 267 aircraft — the largest non-military air force in the world — and four unmanned Predator B aircraft to watch the border.

And now all cargo is screened before it gets to the U.S., and shipments deemed high-risk are checked upon entry.

According to federal immigration statistics, fewer people are trying to cross into the U.S. and arrests of illegals dropped 20 percent last year and 8 percent in 2006.

Despite the improvements, critics say the U.S. is not close to securing the border, and not close to being safe.

"It would be so easy to come in here," said Jim Gilchrist, founder of the Minuteman Project, a group of volunteers who patrol the Canadian and Mexican borders. "There is literally no protection. There are some border patrol agents, but at night, I could assure you I could go into Mexico and return to the United States carrying a suitcase nuke."

As much attention as the southern border gets, experts say the northern border may prove more dangerous.

Canadian intelligence estimates 50 known terrorist groups operate in that country and that some have entered the U.S. In 2006, the federal government's General Accountability Office repeatedly videotaped its agents crossing into America from Canada without going through immigration checkpoints.

Gene Davis, a former Border Patrol sector chief who consults on border security issues, said we must secure both borders and make less attractive the thing that draws people here: jobs.

"It's a national sovereignty issue," he said. "And if we cannot control our borders, if we cannot control who comes here, we're never going to have national security."

The hundreds of thousands of illegal immigrants who cross the border every year, for jobs and other reasons, prove that the border is still porous. And critics say some of the technology set up to prevent them is not ready for prime time.

Project 28, a series of 100-foot-tall towers with radar, high-definition cameras and other sensor equipment that was intended to create a virtual fence stretching across Arizona's border with Mexico, didn't work as planned and has been delayed for over a year. Creation of a physical fence in Texas has been delayed by private-property and environmental concerns and the debate over its funding in Congress.

Ahern said progress has been made, but there needs to be a continuing commitment to the investment America has made to border security.

"We need a steady funding stream as we move into the next few years to make sure we're able to sustain the investments we've made in technology and infrastructure along our borders," Ahern said.

FOX News correspondent Dan Springer contributed to this report.