Four Canadian border crossings were shut down Sunday as about 60 of Canada's unarmed border guards walked off the job after they were warned that a person classified as "armed and dangerous" may be headed into Canada.

The walkouts — permitted when the guards perceive threats to their personal safety — began mid-afternoon and stalled northbound border traffic for hours.

Canada's national CTV News said the walkout was triggered by a report from Homeland Security officials in the U.S. who told the Royal Canadian Mounted Police that a suspected killer from California, considered armed and dangerous, might try to enter Canada.

Willie Hicks, branch chief for trade operations on the U.S. side, said the Canadians told him only that they'd been alerted that such a person may be en route.

Faith St. John, an official with the Canada Border Services Agency, told CTV that management staff has been called in to deal with the huge backlog of cars and trucks waiting to cross into Canada. "As more managers arrive, more lanes will be opened," she said.

By about 8:30 p.m., southbound traffic was flowing smoothly and northbound traffic was beginning to move, Hicks said.

Affected crossings were at the Peace Arch Park, for Interstate 5 and State Route 99; the Blaine truck crossing, for state routes 543 and 15l; Sumas, for state routes 9 and 11; and Lynden, for State Route 539.

There have been several such walkouts since the terrorist attacks of Sept. 11, 2001.

In August, Canadian Prime Minister Stephen Harper said Canada's border guards will be armed starting in September 2007.

Harper said it would take 10 years to fully implement the plan. The government will have at least 150 officers with sidearms deployed by the end of March 2008, Harper said at a news conference at one of the border crossing south of Vancouver, British Columbia.