Despite efforts to seal U.S. borders against terrorists, the threat may already be here, according to counterterror officials worried about Americans seeking to attack the country from the inside.

Whether Al Qaeda sympathizers, abortion clinic bombers or a single eco-terrorist waging violence on behalf of the environment, U.S. authorities say a small number of fellow citizens pose as much a threat to the nation as foreign terrorists.

Coming on the heels of attacks in Madrid and London by self-organized, ad hoc cells of homegrown extremists, the threat was highlighted anew last weekend with Canada's arrests of 17 Muslim Canadians charged with running a terror ring from Ontario.

"We still have to look at Al Qaeda as an organization, even though its ability to operate has been somewhat limited," Joseph Billy, acting assistant director of the FBI's counterterrorism division, said in an Associated Press interview Monday.

"Then, on the other hand, you have maybe a lone wolf, maybe small groups, maybe a small cadre of people who may be working within their own country to want to plan for an act of terrorism in order to further their own objective," Billy said. "I don't think one is any more likely than the other."

A University of Maryland database compiling information on terror incidents worldwide since 1970 concludes that one of every seven attacks is carried out by a homegrown extremist. A January 2005 priority sheet for the Homeland Security Department listed domestic Islamic extremist groups and eco-terrorists as top threats.

Two Georgia men facing federal terrorism-related charges in the United States — Ehsanul Islam Sadequee and Syed Haris Ahmed — are linked to the Canadian case for allegedly recording "casing videos" of the Capitol and other potential targets in Washington. Ahmed's lawyer, Jack Martin, said there may have been some connection between his client and the suspects, but he insisted it wasn't part of any terrorism plot.

Responding to the arrests, the U.S. Border Patrol stepped up vigorous inspections of traffic entering the country from Canada and put agents on overtime on high alert along the 4,000-mile border.

"There is definitely a ramp-up of operations specific to this past weekend due to the activity," Border Patrol Chief David Aguilar told reporters in Washington.

Fewer than 10 percent — an estimated 1,000 of 11,500 — of the nation's Border Patrol agents are deployed to the Canadian border, said Customs and Border Protection spokeswoman Suzanne Trevino. The bulk of the nation's agents are stationed along the porous Mexican border, where more than 1.1 million illegal immigrants last year were stopped after entering the country.

The Bush administration has made stemming immigration traffic from Mexico a top priority and is sending 6,000 National Guard troops to assist the Border Patrol at the southern border by the beginning of August.

Aguilar, appearing with National Guard chief Lt. Gen. Steven Blum, said the Border Patrol has no plans to ask for reinforcements of reserve troops to the northern border to mirror security efforts down south.

"Our efforts are all threats along all borders of our nation," Aguilar said. "So we're concentrating not just on the southern border. ... That does not by any means mean we are giving up or abandoning the northern border."

But Hartford, Vt., Police Chief Joseph G. Estey said the comparatively lax security along the northern border has long been acknowledged by local police.

"It's not being paid a lot of attention to," said Estey, the immediate past president of the International Association of Chiefs of Police. "That's something everybody recognizes, but it's also a pretty vast area to try to establish some real stringent security controls. It's a real challenge."

Estey said police look for warning signs for homegrown extremists, such as a large cache of automatic weapons or "something appearing to make a little bigger application to it than we might expect on a local level."

Gary LaFree, director of the National Center for the Study of Terrorism and Responses to Terrorism at the University of Maryland, said homegrown terrorists may only have loose contact with Al Qaeda or other established groups but do most of their networking over the Internet.

"It's not like they were all trained in camps of Afghanistan or Iraq," LaFree said. "No matter how extreme your belief, you can find somebody in the world on the Internet who shares it, and you also can get technology very rapidly. You get these virtual groups coming together more rapidly, and the faster the people operate, the less time you've got to intercede."