High above the rugged border, an unmanned Predator B drone equipped with night-vision cameras and cloud-piercing radar has scanned the landscape for signs of smugglers, illegal immigrants or terrorists.
Armed agents checked the identification of border crossers while radiation sensors and other devices monitored vehicles entering by road. Soon, a network of telescopic and infrared video cameras mounted atop 80-foot metal towers will rise above key locations.
The beefed-up border security is not taking place along America's chaotic southern border -- riven by drug smuggling, gun running and illegal immigration -- but rather, its traditionally boring northern boundary with Canada.
The changes have jarred communities along the 3,987-mile frontier, the longest undefended border in the world.
"Those of us who grew up here never considered it to be a border," said Bernadette Secco, a communications consultant on the Canadian side of Niagara Falls who sometimes dines or shops in the U.S. three times a day. "We're neighbors, not terrorists."
The U.S. has increased security along the Canadian border since the Sept. 11 attacks. But changes are coming more quickly now, driven by fears of terrorists exploiting the relative quiet of the northern border and complaints that the U.S. has been disproportionately soft on Canada.
Homeland Security Secretary Janet Napolitano made the get-tough policy clear in recent comments.
"One of the things that I think we need to be sensitive to is the very real feeling among southern border states and in Mexico that if things are being done on the Mexican border, they should also be done on the Canadian border," Napolitano said at a March conference in Washington on border issues.
"In other words, we shouldn't go light on one and heavy on the other," she said.
Because Mexico, Canada and the U.S. share "one continent" -- as well as the North American Free Trade Agreement to promote trade and investment -- the secretary said, "there should be some parity there."
Before February 2008, the northern border was so open that an oral declaration of citizenship was sufficient to enter the United States.
Starting June 1, however, U.S. authorities will require anyone crossing from Canada to present a valid passport or a secure travel ID card. That has prompted protests from some residents along the border, who say a way of life is ending.
"It was so easy, so habitual, for so long," said Catherine Schweitzer of Buffalo, N.Y., chairwoman of a regional historic preservation group. "Now suddenly there's a gate. And all these new restrictions. And guards with guns. It's scary."
Unlike the Mexican border -- which is half as long -- no drug war or chaos rages in the north. Arrests and drug seizures last year totaled less than 1% of those down south.
"It's a whole lot quieter up here," said Azel J. Price, a Border Patrol agent in Buffalo who worked for seven years in Yuma, Ariz.
That's not to say that border authorities aren't looking for signs of trouble.
On a recent afternoon, an 18-wheeler with Canadian plates set off a radiation alarm as it crossed the Peace Bridge to Buffalo, the northern border's busiest crossing.
The driver was ordered to pass another monitor and park in an inspection bay. A metal arm swept the truck's top and sides and produced a gamma ray image of the cargo. A guard used a hand-held device to identify the offending isotope. Another grabbed bolt cutters and snapped the truck's rear lock.
He quickly found the problem in boxes stacked inside: scented kitty litter. Clay in cat litter emits harmless radioactive traces of uranium, thorium and other natural elements.
"We see this all the time," said Brad Kovach, a U.S. Customs and Border Protection officer, as he peered in the trailer. "Sludge, medical waste, tiles -- we get hits maybe a dozen times a day. It's not a problem."
Then there's bingo. Senior patrons of a popular bingo parlor on the Canadian side regularly trigger the alarms on the way home; they may be carrying pacemakers or have had other medical procedures in which isotopes are used.
The quiet may be deceiving. U.S. officials warn that, at least in theory, a terrorist attack is more likely to emerge from Canada than Mexico.
After all, Ahmed Ressam -- the "millennium bomber" convicted of plotting to blow up Los Angeles International Airport -- was stopped coming off a ferry from Canada in late 1999 with a carload of explosives.
A U.S. Customs and Border Protection report to Congress last year noted a "significant concern" that extremists could slip across the northern border. It cited the "undisputed presence in Canada of known terrorist affiliate and extremist groups," including Hezbollah, Hamas and the Armed Islamic Group of Algeria.
Contraband and illegal immigration from Canada also pose challenges. In a November report, the U.S. Government Accountability Office noted "networks of illicit criminal activity and smuggling of drugs, currency, people and weapons between the two countries."