Canada's political and military leaders would have a tiny and rapidly shrinking window of time to decide whether to shoot down a civilian airliner in the event of a Sept. 11-style attack on this country, according to secret documents obtained by CBC News.

The plans, inadvertently provided to CBC under Access to Information, lay bare the complexity of the decisions military and civilian officials would be compelled to make in a scenario like the one 16 years ago, when four airliners were hijacked and aimed at buildings in Washington and New York.

CBC News has opted not to reveal any details in the plans that might compromise national or military operation security.

The classified documents detail the chain of command for responding to such a threat in Canada and outline what might occur if one or more links in the chain of command are unreachable.

That includes the designation of an "engagement authority," the person who would ultimately decide to shoot down a passenger-laden civilian airliner thought to have been overtaken by hijackers. The authority would rely heavily on a close circle of military and civilian advisers who would provide the crucial information needed to make that final call.

The documents present scenarios in which that authority would lie with the chief of defence staff, who would have scant minutes to make a decision.

Chief of Defence Staff Gen. Jonathan Vance said there is not a day that he doesn't feel the weight of that responsibility.

"We're trained to deal with it," Vance said in an interview. "We've got procedures that we practise, over and over and over again, to make certain that we can think as clearly as we can in a crisis situation."

Documents outline potential attacks

The documents were part of a briefing presented to Vance in 2015 to prepare him for taking on the role of Canada's top general. They contain several sections and full pages that appear to have been marked for redaction, but the information in those sections is still clearly visible.

You don't get a second chance to react. You just have to react, and you become very dispassionate, very cool, some would might even say icy, because you could not get caught up with emotions of the moment - David Collenette , former transport minister

The plan is known as Operation Noble Eagle, a North American Aerospace Defence Command (Norad) operation initially developed after the Sept. 11 attacks in the United States to protect against similar attacks within North American airspace.

But circumstances have changed in the years since those attacks, including the kind of threats security agencies are dealing with.

The documents outline two scenarios. The first refers to an air attack from "a known terrorist group that we are in an armed conflict with." The second was a later addition and refers to a scenario in which the military would assist law enforcement agencies such as the RCMP with "a more traditional hijacking scenario, where a disgruntled individual may want to take out his aggressions on a company headquarters or specific person(s), or even the government."

Quick decisions

Phil Gurski, who spent 30 years working as an intelligence analyst at CSIS, said lone actors present a unique challenge for intelligence agencies because they are difficult to track.

"It's really hard to run human sources against a single individual," he said.

But he said these cases are why intelligence services need to adapt to changing circumstances and continue to have sources that can alert them to any chatter within their circles about possible attacks.

In 2001, Transport Minister David Collenette, shown speaking on a cellphone in Montreal just after the Sept. 11 attacks, had to decide what to do about incoming flights as the U.S. shut down its airspace that day. (Patrick Sanfacon/Canadian Press)

It's not a surprise that military, police and government officials are preparing these kinds of plans in a post-Sept. 11 world, but the stark details included in the documents drive home the cold calculations and details involved in such a scenario.

The briefing includes notes about debris fields and the need to minimize civilian casualties should a commercial airliner be shot out of the sky.

It also describes the brief amount of time available to shoot down a plane, a scenario Canadian officials were confronted with on the day of the Sept. 11 attacks.

U.S. officials and then prime minister Jean Chrétien made the call to authorize a Korean Air passenger jet to be shot down if necessary, after it showed signs it might have been hijacked.

American F-15 jets were scrambled and escorted the passenger jet as it landed in Whitehorse. But the order had been clear: if the jet didn't co-operate and land on its own, the military was authorized to shoot it down.

David Collenette was Canada's transport minister on Sept. 11, 2001. After the United States grounded all flights in its airspace, barring incoming flights from landing on U.S. soil, he faced a decision of his own.

There were hundreds of flights in the air coming from Europe and it wasn't clear whether any had been hijacked.

Collenette, along with his deputy minister, ordered those flights to land in Atlantic Canada before they could get to large population centres such as Toronto and Montreal.

"We just had to get them down," he said.

'Never any guarantees'

Collenette said there is only one opportunity to make the right decision in these kinds of situations.

"You don't get a second chance to react. You just have to react and you become very dispassionate, very cool, some would might even say icy, because you could not get caught up with emotions of the moment," he said.

Phil Gurski, a former CSIS analyst, says there are risks for decision-makers during a potential attack. 'There are never any guarantees, with few exceptions, that any piece of information is 100 per cent accurate,' he says. (Phil Gurski)

The burden lies not just with the decision makers, but also those tasked with providing the crucial information needed to make those choices.

As an intelligence analyst, Gurksi was one of those responsible for feeding relevant and accurate information to the people making critical decisions in the aftermath of the Sept. 11 attacks.

"You're only as good as the accuracy of your own information," he said.

In a crisis situation, where the need for intelligence is urgent and minutes are precious, Gurksi said, you do your best, but developing new sources or finding that key piece of information is sometimes next to impossible — and comes with risks.

"There are never any guarantees, with few exceptions, that any piece of information is 100 per cent accurate," he said.