Concerns with smuggling and terrorism have led to heightened security along U.S borders. Mike O'Sullivan reports from Seattle.

The U.S. border with Canada, 6,400 kilometers from the Atlantic to the Pacific oceans, has been described as the world's longest undefended international boundary.

The U.S. southern border with Mexico has hundreds of kilometers of high fencing, with more planned, but there is little to mark parts of the U.S. northern border.

A small fence is all that separates the U.S. town of Blaine, Washington, from a Canadian highway.

Officials along the northern border are trying to balance the need for protection with the need to speed the cross-border flow of goods and people.

Concerns with terrorism have led to added layers of security at border checkpoints, where people crossing over are photographed and questioned.

Customs and Border Protection agents check drivers, passengers and cargo. They are looking for contraband, undeclared goods, and people, including terrorists, trying to enter the country illegally. Supervisory agent Larry Nichols says the screening process is both an art and a science.

"Scientifically, we have better technology than we did a few years ago," he said. "The art - we do have officers that are very intuitive, and they are trained to detect deceit, illicit responses."

New technology is also helping safeguard the border.

In this area near Seattle, a network of cameras, seismic motion detectors and banks of computer monitors help agents keep track of the border. Employees monitor video screens, and dispatch agents where they are needed.

Audrey Block lives meters from the border, where things are quiet now. She says before the security upgrade, this area was popular with Canadian drug smugglers.

"During the daytime, we would be out here working in the yard, and a car would race to the barricade, and a car would stop over there, and they would throw the drugs across, and they would be gone," said Audrey Block. "So we were aware we were at the border at that time. But now that the Border Patrol is so active and the cameras are there, we are very comfortable."

Heightened security is also in place at Seattle's port, although critics say the U.S. trading infrastructure remains vulnerable, from its ports to its highways and its railroads.

Officials at the border say they are working to strike a balance, extending security while maintaining an open border with Canada.