The warning came from Jack Hooper, CSIS deputy director of operations, in May 29 testimony before a Canadian Senate defense committee. He told the committee that, since 2001, some 20,000 immigrants from the Afghanistan/Pakistan region have entered Canada. And said Hooper in what passes for Canadian understatement: "We're in a position to vet one-tenth of those. That may be inadequate."

What that means for the United States is obvious. Our northern border must be guarded more closely than ever before in history. Indeed, it has been, for a century, an unarmed, unpoliced border.

Hooper said, according to CBC News, that young Canadians with immigrant backgrounds are: (1) becoming radicalized through the Internet and (2) seeking targets within Canada itself, not abroad.

"They are virtually indistinguishable from other youth," said Hooper. "They blend in very well to our society, they speak our language and they appear to be--to all intents and purposes--well assimilated. They look to Canada to execute their targeting."

Hooper pointed out that the men responsible for the London 2005 subway bombings were from immigrant families. He testified ominously: "I can tell you that all of the circumstances that led to the London transit bombings, to take one example, are resident here and now in Canada."

A Canadian resident, trained in Afghanistan and at one time living in Vancouver, British Columbia, played a key role in the August 1998 al Qaeda attack on the U.S. Embassy in Nairobi. He was the one who trained the embassy bombers.

Even more ominous was Hooper's admission that the CSIS could vet only about one-tenth of the immigrants from Afghanistan and Pakistan. In other words, apart from bare details, which themselves may be spurious, Canada knows nothing--not even current addresses--of approximately 18,000 immigrants from that part of the world who have arrived just in the last five years. According to a report in the Toronto National Post, Hooper admitted that his agency was troubled about its inability to track these immigrants.

That Canada may be a terrorist target is a distinct possibility. Canada has about 2,300 soldiers deployed in Afghanistan on a mission that was recently extended until 2009. And as Hooper noted in his testimony, "Canada has been named on several occasions as one of six Western 'target countries' by al Qaeda leaders, most recently last summer."

But there is now a new and startling concern: Canadian-born fanatics, whose numbers are on the rise. Said Hooper about these nontraditional votaries of Islamist terrorism: "We have cases of white, Anglo-Saxon Protestants converting to the most radical form of Islam. These are people who blend in with us and our neighbors." Hooper said the CSIS does a good job at containing threats it uncovers, but what about those threats it does not uncover?

"We stay up at night," he said, "worrying about the threats we don't know about, and we always used to work on a ratio of ten to one. For every one we knew, there were probably ten out there we didn't. I worry that the ratio has increased."

A new and conservative government rules Canada today. The U.S.-Canada border should be a top priority on the binational agenda when both sides meet again.

Arnold Beichman is a research fellow at the Hoover Institution, Stanford University.