It was as frightening as a Halloween trick can be: It happened in broad daylight. It was caught on camera. The government says it really happened.

On Oct. 31, 2006, a covert agent of the Government Accountability Office (GAO) pulled to the side of a Canadian highway that runs along the U.S. border.

Greg Kutz, GAO's managing director of forensic audits and special investigations, explained what happened in a May 16, 2008, report to Senate Finance Chairman Max Baucus. The report summarized GAO's efforts (from 2003 to 2007) to covertly test the effectiveness of U.S. border security.

The covert agent who approached the U.S.-Canada border on Halloween 2006, according to the report, had just received a signal via cell phone from another agent on the U.S. side.

He started walking south, holding a red duffel bag that carried simulated radioactive material. People on both sides of the border saw what he was doing. Someone on the American side called the Border Patrol.

The GAO, meanwhile, thoughtfully videotaped this and similar incidents, and has posted a highlight tape online.

The tape shows a man loading multiple canisters of simulated radioactive material into a conspicuous red duffel bag. It then cuts to clips of a man carrying the duffel bag away from an SUV, walking past a small obelisk marking the U.S.-Canada border and moving into some brush on the U.S. side.

Another clip shows a man carrying the duffel bag as he walks into the United States past a risible black-on-yellow sign that reads: "Illegal Border Crossing/Entry Prohibited/Activities May be Monitored/Violators Will Be Arrested."

A third clip shows a man with the duffel bag walking into the United States past one of the border-marking obelisks, later cutting to a clip of him loading the duffel bag into the back of an SUV.

In the first incident -- where someone on the U.S. side of the border actually notified the Border Patrol about the man carrying the duffel bag -- the Border Patrol did not catch the intruder.

Nor did they catch him in the other two incidents -- where no one notified the Border Patrol.

That's not the end of the story. It could happen on the Mexican border, too.

Another incident on the GAO's highlight tape shows a covert GAO agent -- wearing a Hawaiian shirt and white shorts -- walking along a "fence" on the Mexican border. The fence appears to have posts that are alternately about 4- and 6-feet high that stand about 5 or 6 feet apart. Continued...