A former director of the Federal Liberal Agency of Canada is adding his voice to the Conservatives' call for a public inquiry into SNC-Lavalin.

Bert Paul, a long-time Liberal, says a non-partisan inquiry that includes testimony under oath is the only way to find out what happened.

"I believe Canadians should know the truth," Paul said in an interview with CTV News Vancouver's Mi-Jung Lee this week.

Paul served as the chair of the audit committee of the Federal Liberal Agency, the chief agent of the Liberal Party of Canada.

The party has been under the microscope for weeks after allegations surfaced suggesting political influence in a corruption and fraud case against the Montreal-based engineering and construction company.

There are already two investigations into the affair, being conducted by the justice committee and the federal ethics commissioner.

But Paul feels there should be a third, non-partisan review of what exactly occurred.

"It would be done independent of the parliamentary system," he said.

"It would have powers associated with the ability to subpoena, so they could request information, they could request documentation. They could also require individuals to come and to give testimony so that Canadians can find out what the facts are."

The allegations of obstruction of justice are serious, he said, and should be dealt with before the federal election.

When asked how he thought the scandal was being handled by Prime Minister Justin Trudeau, Paul said most Canadians find it perplexing.

"The question is, what did he know and when did he know it? I think Canadians are frustrated, as am I, because I believe we have not had a full and independent investigation of the issues, and I think we owe it to the Canadian public to do that," he said.

New evidence from former attorney general Jody Wilson-Raybould regarding SNC-Lavalin is set to be made public Friday.

Additional documents include copies of emails and text messages as well as a new written statement, which Wilson-Raybould is providing to supplement testimony she gave one month ago. Read a recap of that testimony on CTVNews.ca.

Paul also spoke out against recent leaks about the appointment of a Supreme Court Justice, calling them an attempt to undermine Wilson-Raybould's credibility.

With an interview from CTV News Vancouver's Mi-Jung Lee