In a B.C. airport at noon on Feb. 22, 2008, a federal bureaucrat toting a thick stack of documents met with Canada's then public safety minister. Taking a break from a series of funding announcements in various cities, Stockwell Day had squeezed in the meeting to review dossiers that urgently awaited his perusal – and signature.

A half-hour later, five foreign-born men were officially deemed to be probable terrorists too dangerous to be allowed to stay in Canada. The so-called security certificates that Mr. Day had signed off on meant the government could immediately detain the men, and eventually deport them to their Middle East homelands, following in-camera hearings at which the government would rely on secret evidence.

In landmark testimony on Thursday, Mr. Day, who is now retired from politics, was compelled to give insights into how Canadian officials exercise the highly secretive – and enormously controversial – security-certificate power behind the scenes.

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Despite years of security-certificate court battles, such direct testimony from a government minister has never been heard until now. The power is not used arbitrarily, said Mr. Day, speaking to the Federal Court in Toronto via video link from British Columbia. He said he signed them only to protect the "men, women and children of Canada" from harm – and that he feared what would have happened had he not done so.

Politicians can be trusted to be duly diligent in reviewing the complex dossiers of classified intelligence, he said, insisting that he frequently grilled intelligence officials about the cases.

"I wanted full disclosure, I wanted total transparency, I wanted no surprises," said Mr. Day, arguing that he studied the cases for months before signing the orders. So much so, he said, that he didn't need to avail himself of a literal truckload of supporting documents made available to him in the week before the orders were signed.

Lawyers acting for one of the security-certificate detainees have their doubts about the process.

In cross-examination, they asked Mr. Day pointed questions about how much he challenged the secret information that had arrived to him from his security officials. They wanted to know what he made of internal memos that suggested that Canada receives much intelligence from foreign spy services known to torture prisoners overseas.

These questions arose in the cases of Mohamed Zeki Mahjoub, one of three security-certificate detainees who continue to fight the process. Branded a threat upon arriving in Canada in the 1990s, Mr. Mahjoub has admitted that he met Osama bin Laden several times overseas, and that he even once operated a large Sudanese farm for the eventual leader of al-Qaeda.

Mr. Mahjoub, who is no longer jailed and has always denied being a terrorist, attended Thursday's hearing. At one point, he chuckled derisively as he watched Mr. Day describe him as a man whose very existence in Canada threatens the population at large.

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Pressed to account for his reasoning in ordering Mr. Mahjoub detained, Mr. Day could only recall the case in broad strokes. He remembered he had been given intelligence information indicating Mr. Mahjoub had been a leader of an Egyptian terrorist faction, and that he had snuck into the country with a fake Saudi passport. But whenever lawyers pushed for finer-point recollections of the intelligence dossier, and how Mr. Day might have challenged it, he frequently answered: "I can't recall."

Five years on, Mr. Day testified he has only vague recollections of information that he said he once "intensely" studied before signing off on the certificates.

The Conservative politician pointed out his pen stroke had merely revived pre-existing cases that the Liberal government had launched, but which were put in limbo after the Supreme Court demanded legal fixes to the security-certificate regime.

The five security certificates that Mr. Day signed in 2008 were renewed just as a year-long grace period threatened to elapse. The Supreme Court had struck down the old law but had given the federal government a year to come up with fixes. Had new certificates not been issued, the detainees in question would have faced no restrictions on their liberties.