Along the 4,000-mile line between Canada and the U.S. mainland, Border Patrol agents tightened security Tuesday with stricter ID checks and more searches of incoming cars.
U.S. officials say it's in direct response to the Canadian arrests. High time, too, says the chairman of the House Homeland Security Committee.
"We saw the extent of the arrests this past weekend with probably more to come, and you realize how porous the border can be, so this is a real concern," says Rep. Peter King, R-N.Y.
The FBI, meantime, is following up on information found during searches of the homes of the Canadian suspects, including names of some Americans they were in contact with. It's not yet clear, investigators say, what those contacts were about.
The FBI remains intensely interested in two Atlanta-area men who met with some of the Canadians last year and may have conducted paramilitary training with them in a rural area north of Toronto.
But what worries the FBI even more about the two, Ehsanul Sadequee and Syed Ahmed, is that both traveled overseas — Ahmed to Pakistan, hoping to get terror training, and Sadequee to Bangladesh, where his family is from.
Several U.S. sources tell NBC News that Sadequee went there to meet leaders of al-Qaida in Bangladesh, a country where the terror organization is still robust and easily approached.
"These are the organizations that still have mosques that are under their control, that still have open recruiting offices, that still have people that you can go to physically and speak to," says Evan Kohlman, a terrorism expert and NBC News consultant.
The Americans met the Canadians and other terror figures overseas in cyberspace, through conduits that law enforcement and intelligence services continue to monitor very actively.