Originally published on Oct. 27, 2017.

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An undercover FBI agent, who played the key role in exposing a plot to derail a Via Rail train outside of Toronto, says he has no qualms about his assignment, despite doctors saying one of the men involved in that investigation was psychotic and suffering from schizophrenia.

His goal was to maim and kill as many innocents as possible. So he knew right from wrong. - Tamer Elnoury, FBI agent

Tamer Elnoury — the pseudonym for an Egyptian-American FBI agent — befriended a Tunisian science student in Montreal named Chiheb Esseghaier when the agency began to suspect that he had connections to al-Qaeda. The RCMP and CSIS would join the FBI in the investigation.

In 2015, Esseghaier's schizophrenia was diagnosed by a forensic psychiatrist during the sentencing phase of the trial.

"In no uncertain terms, Chiheb Esseghaier was an absolute threat to society. His goal was to maim and kill as many innocents as possible. So he knew right from wrong," Elnoury tells The Current's Friday host Laura Lynch, who covered the trial.

Raed Jaser, left, and Chiheb Esseghaier, right, were sentenced to life in prison in 2015. Both men are appealing their verdicts. (Frank Gunn/Canadian Press)

He had the intent to kill. - Tamer Elnoury

"He rationalized scientifically, religiously, in his warped way," he explains.

"Even though it is technically 'haram' — which means forbidden to hurt people — it's a necessity of war based on what we are going through. That is not the mind of a madman, that has a mind of a lucid thinker who is absolutely planning and plotting, and he had the intent to kill."

Elnoury has written about the Canadian case in his new book, American Radical: Inside the World of an Undercover Muslim FBI Agent.

For security reasons, his voice was disguised in The Current's interview.

Chiheb Esseghaier, who is now on anti-psychotic medication, is appealing his life sentence on the grounds that his mental state at the time of the trial made his conviction a miscarriage of justice.

What worries me professionally and personally is missing one — I don't want to miss a single one - Tamer Elnoury

Despite the convictions, Elnoury still wishes Canadian law enforcement had not shut down the seven-month long investigation when it did.

"I felt there was so much more meat on the bone," he tells Lynch.

The agent recently spoke to the CBC's The Fifth Estate in an exclusive Canadian television interview. The full interview will be broadcast this Friday on CBC Television. 1:15

During the investigation, Esseghaier mentions that an al-Qaeda sleeper agent was operating in the U.S.

Elnoury says there hasn't been a day since April 22, 2013 (when the Canadian government stopped the investigation), where he hasn't thought about him.

"What worries me professionally and personally is missing one — I don't want to miss a single one," he says.

"I believe there is evil out there everywhere, and I am thankful and happy and proud to be a part of an organization that dedicates all of its resources to stopping that evil every day."

Find out more about this story at CBC TV's The Fifth Estate.

Listen to the full conversation above.

This segment was produced by The Current's Howard Goldenthal.