Canadian counterterrorism agents were investigating the possibility of an al-Qaeda plot to blow up targets in Ottawa when they began a probe that would lead to the detentions of Maher Arar and several other Canadian Muslims half a world away.

Police have never revealed the reasons for the 10-month jailing of Mr. Arar, who denies any involvement with al-Qaeda and is fighting for a public inquiry in his case. But information obtained by The Globe and Mail points to a series of events that started just before the Sept. 11, 2001, attacks in the United States.

In late August, 2001, U.S. border guards discovered a single sheet of paper — a schematic map of Ottawa marking government buildings and nuclear research facilities — in an 18-wheeler driven by a man named Ahmad Abou El-Maati.

During eight hours of grilling, the 39-year-old Toronto truck driver, a devout Muslim, denied owning the map. But when he returned to Canada, he was dogged by counterterrorism agents. Months later, search warrants were executed at seven Ottawa locations as RCMP officers looked for explosives and diagrams of government buildings.

And when Mr. El-Maati travelled to Syria in November, 2001, he was immediately jailed as a terrorism suspect — the first in a series of Canadian Muslims to face such a fate. Locked up for more than two years, he was freed from prison this week.

His Middle Eastern captors no longer regard him as a suspect. And in Canada, where no one has ever been charged with a crime as a result of the investigation, the probe is being described as a necessary precaution.

“I would say the chance of it [a plot] being likely was 35 per cent, and 65 per cent not, but it's definitely something you have to follow up on,” a government source said Thursday. “You've got to make sure it's not.

“I know a lot of people don't believe this, but we are involved in a war — it's called a war on terror. And in any kind of war, innocents are hurt.”

This past summer, investigative reporter Seymour Hersh stated in The New Yorker magazine that “the Syrians also helped the United States avert a suspected plot against an American target in Ottawa.” On what basis Syrian officials would claim this is not explained.

Within the span of a year, a series of Canadian citizens ended up behind bars in Syria.

Mr. El-Maati was first, in November, 2001. Then in May, 2002, it was Abdullah Almalki, whose Ottawa residence was searched in the RCMP raids while he was in Malaysia. A few months later, Arwad Al-Bouchi, another Syrian from Ottawa who had by then moved to Saudi Arabia, was also arrested as he entered Syria.

Mr. Almalki and Mr. Al-Bouchi remain in jail. Each was arrested while travelling to Syria on what they said were visits to family members.

In September, 2002, Mr. Arar was arrested in New York and deported to Syria. U.S. sources have said the Mounties had placed him on a watch list. U.S. agents who intercepted him accused him of being an al-Qaeda agent, and showed him a lease he had signed with Mr. Almalki as proof.

Mr. Arar spent 10 months in jail. He has said he had fleeting encounters with Mr. Almalki in Canada, but did not know him. Through a spokeswoman, Mr. Arar has also said he once bumped into Mr. El-Maati in an auto garage, but that he did not know him either.

Suspicions surrounding Mr. El-Maati were also raised by the fact that the U.S. Federal Bureau of Investigation is looking for his brother, Amro, after his citizenship papers were found in Afghanistan.

But the map seems to be key to the truck driver's ordeal, and he has said as much to people who knew him before he left Canada.

“He showed me the map. And the map had literally all kinds of government installations,” said a man who used to know him. “If I was a border person and I saw this map with a Middle Eastern-looking person and all these nuclear sites and all these government installations I can understand why they said, ‘Well, hey pal, what are you doing?' And apparently they really grilled him. When he came to me he was nearly in tears he was shaken up so bad about it.”

Mr. El-Maati left Canada, telling friends and family the map was not his and that he was going to leave the suspicion behind and go be with the young wife he met in Syria a few months earlier.

He had told friends that CSIS was following him and scuttling his attempts to bring his wife to Canada. At the same time, sources say, counterterrorism agents were telling Mr. El-Maati's associates that he had been to Afghanistan and they wanted to orchestrate a sting operation against him.Mr. El-Maati was arrested when he stepped off the plane in Syria. The country transferred him to Egypt, which did not act on several court orders for his release him until it let him go this week. Just why they decided to do so now is unclear.

Mr. El-Maati's father in Toronto and mother in Cairo both said Thursday that their son is happy to see friends and family again, and wants his privacy to be respected. They say he was treated well in custody and will say nothing that might be construed as critical of Egypt.

Thursday, at the Toronto airport, Mr. El-Maati's imam — Aly Hindy of the Salaheddin Islamic Centre in Scarborough — said that recent conversations he had with Egyptian officials may have helped to free the jailed man.

Mr. Hindy was at the airport to greet Muayyed Nureddin, another Canadian jailed in Syria who was freed this week after being detained there for a month. Mr. Nureddin thanked Canadian officials for working to free him from Syria where he was arrested in December. He is not thought to be linked to the other Canadians jailed in Syria. Friends say he was put under scrutiny by CSIS before his capture.

In fact, Muslim and Arab groups are beginning to push for a broadening of any public inquiry into the Arar case to examine why information gathered in Canada seems to be playing a recurring role in the detention of Canadians abroad.

The RCMP said Thursday they are aware of Mr. El-Maati's release, but will not comment further. Nor will the Mounties comment on the seven search warrants executed around Ottawa, which have been sealed. A lawyer representing Mr. Almalki has seen those warrants, but cannot comment on them.

A CSIS spokeswoman said that the agency gathers intelligence lawfully, in accordance with its mandate, and has information-sharing arrangements with many countries, but cannot reveal how it may swap information with Syria.