STRATHROY, Ontario (REUTERS) - The man killed during a Canadian police raid at his home in Ontario on Wednesday (Aug 10) was a supporter of Islamic State in Iraq and Syria (ISIS) who was in the final stages of attacking a major urban centre with a homemade bomb, police said on Thursday (Aug 11).

Police raided the home of Aaron Driver in the small town of Strathroy after receiving credible information, including a"martyrdom video," from US authorities that he planned what could have been a "dreadful" attack, the Royal Canadian Mounted Police (RCMP) said.

Driver died after he detonated an explosive device in the backseat of a taxi as police closed in, the RCMP said at a news conference in Ottawa.

The RCMP had said on Wednesday that he was fatally shot by police, but at Thursday's news conference the police could not say if he died as a result of the detonation, or as a result of being shot by officers.

"The outcome if we have not been able to apprehend him, based on his actions when he was confronted, could have been significantly more dreadful," said Deputy Commissioner Mike Cabana. "It was a race against time."

The incident was the first security test for Prime Minister Justin Trudeau, who was elected in October 2015 and who in February fulfilled a campaign pledge to withdraw Canada from the combat mission against ISIS and to increase its mission training local fighters against the group in northern Iraq.

The video provided by the US Federal Bureau of Investigation allowed the RCMP to identify Driver and raid his home in Strathroy, about 225km south-west of Toronto. In Washington, the FBI said it gave the RCMP "actionable threat intelligence".

In the video, which was shown at the news conference, a man in a black balaclava cites a phrase from the Quran, refers to crimes against Muslims and pledges an imminent attack on a Canadian city.

"Oh Canada, you received many warnings, you were told many times what would become of those who fight against the Islamic State," the man says in the video, pledging allegiance to the militant group.

The video indicated that the attack was planned for the next 72 hours, during rush hour. Police said there was no indication that Driver, a 24-year-old Muslim convert, had any accomplices in his plans.

Islamic State media said Driver was its "soldier," the Site monitoring service said on Thursday.

"If he had gotten out of that residence before we got there, the scenario would have ended a lot differently," said RCMP Assistant Commissioner Jennifer Strachan.

Driver, who also used the alias Harun Abdurahman, was arrested last year for openly supporting the militant Islamist group ISIS on social media.

He had not been charged with a crime. But in February he was placed on a peace bond, a court order that restricted his movements, required that he stay away from social media and computers and not have contact with ISIS or similar groups.

Police said at the news conference that Driver had not been under constant surveillance, but had been monitored.

Strathroy is a town of about 21,000 inhabitants in the heart of Ontario's farmland. Driver's house was on a tranquil street lined with detached two-storey homes, near a baseball field and a swimming pool.

WARNING TO TRANSIT OPERATORS

Public transit operators in Toronto, Canada's largest city, were warned by police of potential security threats hours before officers killed Driver, they said on Thursday.

The Toronto Transit Commission (TTC), which serves the city, and the regional operator GO Transit confirmed they were contacted by police early on Wednesday.

A representative from the local Leo's Taxi Transportation Ltd. said a cab had been dispatched to Driver's address on Wednesday night at the time of the police raid.

The representative, who declined to be identified, said the taxi driver was injured in the incident, but has since recovered.

"He's shaken up a bit, but he's OK," the representative said. "It was a shock, right?"

Aaron Driver was a troubled child who converted to Islam in his teens some time before his support for ISIS attracted the attention of Canadian police.

Amarnath Amarasingam, a researcher at Dalhousie University who studies the paths individuals take towards extremism, said on Twitter that he had received an email from Driver on April 17 that expressed his satisfaction with his life.

Amarasingam suggested that a call to action from an ISIS spokesman and recent attacks by other lone actors pushed him from holding radical views to acting on them.

In 2014, Canada was stunned by two deadly attacks that police said were the work of home-grown radicals and that led to tougher new anti-terrorism measures. A gunman killed a soldier at Ottawa's national war memorial before launching an attack on the Canadian Parliament in October 2014 while, in the same week, a man ran down two soldiers in Quebec, killing one.

Driver had expressed support for the Parliament gunman, Michael Zehaf-Bibeau, on Twitter, which was how he first came to the attention of security officials.

Driver's former lawyer, Leonard Tailleur, said he was surprised by the incident as he had had an expert assess Driver and had found no signs of violence, despite his sometimes extreme views.

In March last year, Canada said it had foiled a plot by a self-proclaimed Islamic State supporter to bomb the US consulate and other buildings in Toronto's financial district.