In an Aug. 2012 letter obtained by CTV News, Alison Stodin, former aide to Pamela Wallin, accused the senator of mismanagement and personal harassment during a dispute over travel claims.

The existence of the letter was first revealed Friday. It was outlined in court documents filed by RCMP investigators, who are seeking to obtain evidence as part of the Mounties' ongoing investigation into Wallin's expenses.

Stodin's letter to Sen. Carolyn Stewart Olsen, who was then a member of the Senate Standing Committee of Internal Economy, helped trigger the audit of Wallin's expenses. It is among a number of documents the RCMP is seeking as it considers possible charges of fraud and breach of trust.

"I have never been subjected to the kind of treatment and disrespect I received from Senator Wallin," Stodin wrote in the letter. "This involved harassing me personally, screaming at me and threatening me."

A veteran executive assistant with 20 years of experience on Parliament Hill, Stodin was hired by Wallin's office in 2012. She interacted with Wallin mostly through phone and email in the six weeks she worked in the office, before writing the letter.

In the letter, she alleges Wallin accused her of double booking flights and not following her orders.

Sources say Wallin's expenses became confusing, and that Stodin and Senate accountants had to go over the claims together, which is when the attacks intensified.

"Wallin terminated my employment on August 22, 2012 stating incompetence, and inability to run her schedule. She also said that she would tell future employers that I was a negative recommendation," Stodin wrote in the letter.

On Feb. 13, Prime Minister Stephen Harper rose in the House of Commons to defend the Saskatchewan Senator he hand-picked.

"I've looked at the numbers, her travel costs are comparable to any other parliamentarian travelling from that particular area, period of time," he said.

Sources told CTV News that Stodin wrote a text message to Harper's former chief of staff Nigel Wright that night to let him know of the issues:

"Wallin's travel expenses are not anything the PM should publicly defend," she wrote. "I worked with her last summer doing her expenses & it was really problematic, in my opinion."

But Wallin's lawyer Terry O'Sullivan said Stodin is nothing more than a disgruntled employee who had "several performance issues."

Those who have known Stodin for years, however, have nothing but praise.

"I knew Alison for a long time, she's very honourable, and hard worker," said B.C. Conservative MP John Duncan.

Jim Armour, former director of communications to Stephen Harper, also backed her up. "She's a very trustworthy person, if that's what Alison says, I believe her," Armour said.

With a report by Daniele Hamamdjian, CTV News