Prime Minister Justin Trudeau says that when the Liberal Party was vetting him as a candidate, he never owned up to the documented cases of blackface in his personal history.

"I never talked about this. Quite frankly, I was embarrassed," Trudeau told reporters in Winnipeg Thursday. "It was not something that represents the person I've become, the leader I try to be and it was really embarrassing."

Trudeau offered no further details on how he navigated the party's process for vetting candidates or how he continued to hide his embarrassing past.

According to the party's rules for vetting candidates, a qualified nomination contestant "has a continuing obligation to disclose to the national campaign chair any information that could impact upon their acceptability as a qualified nomination contestant or as a candidate of the party.

"Failure to disclose such information constitutes non-compliance with these rules … and may result in the disqualification of a qualified nomination contestant."

Trudeau said he did not tell his staff about his history of blackface until Time magazine told him it was about to reveal that in 2001 he attended an Arabian Knights gala at a private school in B.C., where he was a teacher, dressed in a turban with his skin darkened. The story included a photo from a school yearbook.

"Ultimately, the call is mine on when to talk to people, when to act on things, and the buck stops with me and I take responsibility," he said.

All parties vet candidates

In an effort to avoid embarrassments like Trudeau's, candidates for all parties are vetted to unearth anything that could possibly derail their campaign or hurt the party.

The Liberal Party says its "Greenlight Committee" scrutinizes each prospective candidate to ensure a clean slate and its rules stipulate that candidates must not have "engaged in any claim, litigation or dispute of any sort which is liable to bring controversy or disrepute upon the qualified nomination contestant or the party."

Kaveh Shahrooz failed to secure the Liberal nomination for the Toronto-area riding of Richmond Hill in the 2015 federal election, losing out to Majid Jowhari, who went on to win the seat.

We don't have people going to campuses. That just doesn't happen. - Michele Cadario

Shahrooz told CBC News that the committee had him fill out an extensive questionnaire detailing his background, views, education and financial information. After that, researchers for the party reviewed his social media history and then conducted a telephone interview.

"Something like an old yearbook? I don't think that would have been caught in that greenlight process," said Shahrooz. "If they don't offer up the information, I don't think that vetting 338 candidates it's possible to go and look at everyone's old yearbooks. At the candidate level it's hard to do."

From vetting to crisis management

Jowhari may have won his seat, but within a year he was forced to apologize for misrepresenting his professional credentials as an engineer during the campaign. While he had qualified as an engineer, he was no longer licensed. To make amends he had to apologize and donate $5,000 to the Ontario Professional Engineers Foundation for Education.

Michele Cadario, the former federal campaign director for Paul Martin, helped oversee the vetting process for Martin's prime ministerial campaign. She said that unless a candidate freely reveals they have an embarrassing incident in their past that would not be picked up with a social media search, credit check or internet search, it's very hard to get a complete picture.

"Unless something got flagged in that research. The idea of tracking down a yearbook or a newsletter or anything that's written, unless it's anything that's like a published article or a published book, or anything like that which would absolutely be read, but we don't have people going to campuses. That just doesn't happen."

Cadario said that all issues that arise in vetting are dealt with on a case-by-case basis. If, for example, someone made a racist comment a year ago on social media, it would be treated differently than a drunk-driving conviction that was decades old and where candidate had worked hard to make amends.

"We have a candidate survey that's fairly exhaustive that they have to go through," she said. " A lot of it is based on asking them what they have been involved in, and what they've done and would be embarrassing to the party. We wanted them to acknowledge it."

Cadario said that when candidates do not reveal past transgressions, such as Trudeau's blackface incidents, and those incidents are revealed after the candidate has been elected, it is no longer about vetting. The issue then becomes crisis management.