Canadian police have foiled an al Qaeda-backed plot to derail a passenger train in the Toronto area in a "a major terrorist attack", officials have said.
The suspects allegedly received direction and guidance from al Qaeda militants based in Iran, but there was no evidence of a link to the Iranian government, investigators said.
Two Canadian residents were arrested in Toronto and Montreal yesterday after a cross-border Canadian-US surveillance operation that began last August.
The plot has no connection to the Boston marathon bombings. But the timing of the arrests - on the day that the sole surviving Boston suspect was charged - will further heighten concerns about terror attacks in North America.
Police did not disclose why they swooped yesterday. The attack was in the planning stages and the men had the capacity and intent to conduct the attacks but the public were in no imminent danger, according to the Royal Canadian Mounted Police (RCMP).
Chiheb Esseghaier, 30, and Raed Jaser, 35, are expected to appear in court today on charges that they conspired to carry out an attack and commit murder in association with a terrorist group. They are not Canadian citizens, but the authorities did not disclose their home countries or how long they have been in Canada.
There were unconfirmed media reports that one was Tunisian and the other came from the United Arab Emirates and that they had not been in Canada for a long period.
During the surveillance operation, the two men were monitored as they cased out trains and railways in the Greater Toronto area, Canada's most populous area. They are thought to have been planning to attack a train destined for New York.
The arrests were executed by a special joint task force of Canadian police and anti-terrorism units. Operation Smooth was conducted with the assistance of the Federal Bureau of Investigation and the Department of Homeland Security in the US.
The Canadian authorities said the operation was still underway and they were last night searching addresses linked to the two men. They are understood to have been tracking the men's movements, email messages, Internet activity and mobile phone calls since receiving a tip-off from the Muslim community in Canada.
"Had this plot been carried out, it would have resulted in innocent people being killed or seriously injured," said James Malizia, the RCMP's assistant Commissioner
"While the RCMP believed that these individuals had the capacity and intent to carry out these criminal acts, there was no imminent threat to the general public, rail employees, train passengers or infrastructure."
Al Qaeda has a long record of identifying public transport targets, including the July 2005 London Underground and bus attacks and the 2004 Madrid train bombings.
The terror faction's affiliate in Yemen has taken on an increasingly active role in co-ordinating international terrorist plots, but this is thought to have been the first time that al Qaeda representatives in Iran have been accused of involvement.
Although Iran is a Shia-led theocracy and al Qaeda is an extremist Sunni group, Teheran has sheltered several leading al Qaeda figures since they were forced out of neighbouring Afghanistan by the fall of the Taliban regime in late 2001.
The Iranians hold most of them under house arrest and monitor their activities but have allowed them to maintain links with al Qaeda operatives in Pakistan. But if Teheran has now allowed them to guide an operation in Canada, that would be an alarming new development for Western counter-terrorism officials.