Officials guarding the vast and mostly unfenced border between the United States and Canada are increasingly concerned that Islamic activists may pay sophisticated organised crime gangs to transport terrorists and weapons across into the US.
In recent years, Canada has become a global hub of organised crime, run by mostly by Asian and motorcycle gangs. They have set up an intricate network to smuggle marijuana, counterfeit goods and guns into the US.
But according to the Royal Canadian Mounted Police (RCMP), there are fears that criminals could use their network to smuggle terrorists across.
Humphrey Hawksley has been to the border to meet the people in charge of keeping it safe.
SECRET TUNNELS AND HELICOPTERS
Sergeant Brian Brasnett is a senior RCMP border investigator. He believes it would be naive to think the gangs would draw a moral line as to what they transport.
RCMP's Brian Brasnett says aircraft can make illicit drops in minutes
"We would hope they don't stoop to the level of dealing in terrorism," he says.
"But it's all about making money and moving a product so that's where we have to concentrate our enforcement efforts."
He showed me an example of the real danger. Three years ago, US and Canadian investigators discovered a tunnel dug 8ft (2.4m) beneath the ground and stretching more than 330ft (100m).
It ran from a large garden hut on the Canadian side under the road and ditch that divides the two countries into the basement of a private house on the US side.
"It was 4ft by 4ft ...with a cart to transport contraband", says Sgt Brasnett. "There would have been a fee for prospective customers who wanted to use it."
How the gangs use tunnels under the border
The US-Canada border runs for more than 5,000 miles (8,000km) through some of the remotest areas in the world. It's often marked simply by a line cut in scrubland or a small obelisk.
One of the busiest border crossings is one the main highway between the west coast cities of Vancouver and Seattle. A motto on an arch there proclaims that the two countries are "Children of a common mother" and "May these gates never close."
Even so, since 9/11, the US has been building what it describes as a "virtual fence" with an array of gadgetry that ranges from radiation detectors for nuclear weapons, to seismic sensors to catch people illegally sneaking across and number plate recognition so that immigration officers know pretty much who you are before you pull up at the booth.
"We ask ourselves, is this person truly who they are," explains Tom Schreiber of the US Immigration and Customs Service. Does this person match a recent intelligence look-at that I need to be aware of?"
Surveillance video of a drop-off shows the smugglers at work
Billions of dollars of contraband cross the border from Canada every year. It's been found in container trucks, ships and in canvas bags strapped to the skids of helicopters.
"They can fly as low as 500ft through the valleys straight down into the United States undetected," says Brian Brasnett. "Sometimes on a flight that only lasts 10 minutes and vehicles are waiting there ready to pick it up."
They call it "cutting sign" - an ancient Indian skill of reading tracks and footprints and learning how the sun reflects off moving earth and plants.
Every agent of the US Border patrol is a trained tracker.
The US agents have a deep knowledge of their terrain
"Quite often it's just going to be a little smudge," says senior agent Lee Pinkerton, squatting down and brushing his hand across the earth.
"Let's say they're running out of water so they're dragging their toe. Or they're jamming their heel or digging that toe when they're pushing off, or something scared them or maybe they seen me coming and they're running."
Agent Pinkerton and his team ride mustangs caught in the wild then broken in, because these horses are especially good at moving through rugged terrain.
"Our primary mission since 9/11 is to stop terrorism," says Agent Pinkerton as we ride towards the Canadian border in their Spokane sector where Washington State meets British Columbia.
"It's not the hi-tech way to do things, but right here it's the only way to do things. Even though we might have satellites and we might have aircraft, there's still the element of the man on the ground that's going to make that final determination."
How trackers protect the border
The horsemen from the US Border Patrol make it their job to maintain a vigil on inaccessible areas. Sometimes they camp out for days on end, following an intelligence tip that someone might be trying to cross.
At the rickety low fence that divides the US and Canada, our patrols meets up with the Royal Canadian Mounted Police, who, ironically, gave up their horses some time ago. About half a mile away is road where vehicles sometimes stop to unload goods to be taken across.
RCMP border enforcement specialist Brian Brasnett says he's seen some unusual tracks. "There were footprints right along the trail," he says. "Can't tell how fresh they were."
Three of our US Border Patrol team peel off the check them out. Sgt Brasnett and Agent Pinkerton discuss setting up night scope along the trail to help them in cutting the sign.
"Satellites can track radio frequency. Radar that can track aircraft. They're all cutting sign in one manner," says Agent Pinkerton.
"We're going to stand on that line. We're going to patrol. We're going to to do whatever it takes to ensure out country is protected. If they want to cross illegally into the United States, not here. Not on my watch."
The three agents returned to report that the tracks were animal, not human. The US-Canada border stretches for thousands of miles of emptiness, but there's a real determination to keep it safe.
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