Conservative leader Andrew Scheer has ordered an independent third-party investigation into the party's handling of sexual-assault allegations against then-Tory-candidate Rick Dykstra during the 2015 election.

The move comes as the Conservative Party faces pressure from within its own ranks to explain why Mr. Dykstra was allowed to run for the party amid reports that he had been investigated by police after a sexual-assault allegation.

"It is difficult to ascertain the facts when sources are speaking anonymously to the media. So to gather these facts, I have instructed the Conservative Party of Canada to retain the services of an outside, independent third party, who can investigate this situation fully," Mr. Scheer told reporters in Ottawa on Wednesday.

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Mr. Scheer said he would release the findings of the investigation to the public. It was not immediately known who would head up the investigation.

"The key is to make sure if there were breakdowns, if there was a lack of a process, if any of these allegations are true – obviously we have to make sure it doesn't happen again in the future. I think we can all agree on that," he said.

He also said the party will strengthen the code of conduct for all party staff, as well as candidates, and will include mandatory training.

Mr. Dykstra, a former Conservative MP, stepped down as president of the Ontario Progressive Conservatives on Sunday hours before Maclean's magazine reported on accusations of sexual assault dating back to his time on Parliament Hill. The magazine said that federal Conservative officials knew about the allegations against Mr. Dykstra when they allowed him to seek re-election in his Southern Ontario riding, although top Conservative operative Guy Giorno told The Globe that he was not aware of the specifics.

Conservative MPs on Wednesday said they wanted to know more about how the party handled the allegations.

"I don't know why the party didn't do anything at that time and they have to answer these questions," Quebec MP Maxime Bernier, who finished a close second to Mr. Scheer in last year's leadership vote, said in Ottawa on his way into the Conservative caucus meeting.

The issue raises concerns about how candidates are selected, said Ontario MP Erin O'Toole, who also ran for the party's leadership and said he was now considering a run for PC leadership in Ontario.

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"We need to make sure there's a system for vetting and dealing with these situations even during an election," Mr. O'Toole said.

Conservative MP Michelle Rempel, who this week said senior officials in her party "should be ashamed of themselves" for how they dealt with the allegations, said she supports Mr. Scheer's decision to refer the matter to a third party.

"It sends a message to the broader community that our party takes these issues seriously, and that we're committed to getting the process right going forward," she said.

Mr. Giorno told The Globe on Wednesday that he was aware police had investigated an allegation about Mr. Dykstra but had no idea it involved a sexual-assault complaint.

Mr. Giorno, national campaign chair during the 2015 election, said that, to the best of his knowledge, there was never a discussion about allowing Mr. Dykstra to run as a Conservative candidate, because the party's war room had no information about any specific allegations and believed the matter had been closed by police.

"I was not aware of specifics of the allegation. I was not aware that the allegation was of sexual assault. I was not aware that the allegation was of any kind of assault. The implication was that the police file was closed because nothing was found. I definitely was not aware that the file was closed at the complainant's request," Mr. Giorno said in an e-mailed statement.

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According to a Maclean's report, a woman who worked for a Conservative MP said that after a night of drinking that ended in Mr. Dykstra's Ottawa apartment, Mr. Dykstra forced her to perform oral sex on him.

The staffer later met with Kym Purchase, then the chief of staff to the party whip, to speak to her about the alleged assault, the Maclean's report said. Ms. Purchase said the woman did not want a file created on the incident, according to the story.

Maclean's, quoting anonymous sources, said Conservative campaign operatives discussed dropping Mr. Dykstra as a candidate but ultimately allowed him to run in the election. He ended up losing his St. Catharines, Ont., riding.

Mr. Dykstra's lawyers later told The Globe he "categorically denies" the Maclean's story, and that Mr. Dykstra wasn't given enough time to respond to the allegations.

The Globe and Mail has reached out to the woman who made the allegations, but she declined to comment.

Mr. Giorno provided The Globe with a letter from a lawyer representing the former staffer. It is dated Aug. 27, 2015, and is addressed to party lawyer Arthur Hamilton, who was representing the Conservative whip's office.

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The letter does not contain information about the allegations, but it does reference that a meeting took place between the staffer and Ms. Purchase.

Mr. Giorno said he did not speak with the lawyer or Ms. Purchase, but spoke to those who had. "I was informed that the matter related to an allegation that the police had investigated and that the police had closed their file," he said.

Mr. Giorno said on Thursday that the initial discussions about the allegations against Mr. Dykstra happened between Aug. 26 and 28, and that he did not learn that the police investigated until Sept. 8.

With a report from The Canadian Press