Canadian security forces thwarted a plot to blow up a rail line between Canada and the United States and will announce arrests on Monday afternoon, police and intelligence agencies said.
U.S. security and law enforcement sources also said the suspects had sought to attack the railroad between Toronto and New York City. Canadian media said two men had been arrested after raids in Toronto and Montreal, Canada's two biggest cities.
Canadian police are holding a briefing about a "national security criminal investigation" at 3:30 p.m. ET (1930 GMT).
The police statement, which gave no details of arrests, said various Canadian security forces had conducted joint operations in the two cities.
A U.S. law enforcement source told Reuters the alleged plot was not linked with last week's Boston Marathon bombings.
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The Canadian Broadcasting Corporation said the operations was conducted with the U.S. Department of Homeland Security and the FBI.
The arrests follow not only the Boston bombings but revelations that Canadians took part in an attack by militants on a gas plant in Algeria in January.
It also recalls the arrests in 2006 of a group of more than a dozen Toronto-area men accused of planning to plant bombs at various Canadian targets. Eleven men were eventually convicted of taking part on the plot.