A top security official has revealed that Canadian authorities felt they had identified a group of terrorists intent on attacking Toronto around Sept. 11, 2001. But because the prospects of prosecution were deemed to be hopeless, the officials adopted a "diffuse and disrupt mode" meant to stop possible attacks by other means.

"There was a conspiracy of eight individuals who had designs to execute an act of serious violence in the Toronto area," Jack Hooper, a long-serving Canadian Security Intelligence Service official, told a Senate committee this week.

Without naming names, he said that it was impossible to arrest the men in question; Justice Department officials who reviewed the case decided there was "insufficient evidence" to bring it to court. So, spies and police tried to counter suspects by making their lives hard in less-conventional ways.

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"At the end of the day," said Mr. Hooper, CSIS's deputy director of operations, "if prosecution is not viable there are other techniques."

His remarks speak to the kind of anti-terrorism methods employed since the 2001 attacks on the United States. Both CSIS and the RCMP are now under scrutiny in judicial inquiries and civil courts, with a series of men jailed overseas accusing the agencies of making end runs around the justice system.

Read the text of Mr. Hooper's remarks, criminal lawyer Barbara Jackman said: "I think this is a reference to A/O Canada," referring to an inconclusive 2001-era probe dubbed Project A/O Canada by the RCMP. Ms. Jackman, who represents several current and former terrorism suspects, said that many of her clients complain of harassment by authorities even though they are never arrested in Canada.

One client, Toronto truck driver Ahmed El-Maati, would seem to be a textbook case of CSIS and RCMP "diffuse and disrupt" techniques.

While Mr. El-Maati has never been linked to any plot against Toronto, those working on Project A/O Canada suspected him of plotting against Canadian targets. Mr. El-Maati, who is now suing CSIS and the RCMP, says in 2001 he was followed everywhere by Canadian agents -- including ones who showed up at his door on Sept. 11 and used up to a dozen cars to follow him in a single day.

Mr. El-Maati, a naturalized Canadian who has admitted to travelling with mujahedeen fighters in Afghanistan in the late 1990s, said he long attracted scrutiny, but it grew intolerable after a map of an Ottawa government complex was discovered in his truck during a routine search at the U.S. border.

He complains he was harassed into leaving Canada, but suffered a much worse fate upon leaving. He was arrested upon arrival in Syria and jailed for two years, and alleges he was tortured and asked questions that could only come from Canadian authorities. Mr. El-Maati says he was forced to confess to a false plot and made to name Canadian associates who were also under investigation by the RCMP.

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The saga of the Syrian detainee is just one chapter of the CSIS/RCMP actions taken immediately after Sept. 11, 2001. One month after the attacks, Hassan Almrei, a Syrian who had also travelled with mujahedeen, was arrested under deportation laws.

While he was never accused of any plot, he was accused of being a security threat because he got a false Canadian passport for another terrorism suspect -- Nabil al-Marabh, an Afghan-trained mujahed from Syria, whose Toronto apartment and downtown photocopy shop were searched by the RCMP after his arrest.

While Mr. Hooper gave few details about CSIS techniques to the Senate committee, he did say that in 2001, the agency's international partners identified several Canadians who had trained with al-Qaeda forces in Afghanistan. He did not elaborate on "diffuse and disrupt" methods but a CSIS spokeswoman said that these methods could include "arrest, interdiction of persons or materiel at border points" and "deportation on security grounds."