The RCMP is set to conduct a criminal investigation into improper spending by three senators whose expense filings were the subject of external audits, CTV News has learned.

Sources say that the Mounties have been investigating the three senators -- Liberal Mac Harb, Conservative Mike Duffy and Independent Patrick Brazeau -- for months, monitoring the ongoing Senate probe and media reports.

They now have copies of the audits by auditing firm Deloitte that were released Thursday. They also have the Senate’s internal economy committee reports ordering that Harb repay $51,482 and Brazeau $48,744 for housing and mileage claims for the period from April 1, 2011 to March 31, 2013.

After saying he "may have been mistaken" when he filled out Senate housing allowance claim forms regarding a residence in Prince Edward Island, Duffy repaid more than $90,000 in claims in March.

Sources tell CTV News that the Mounties suspect laws may have been breached and a criminal investigation may be warranted.

The RCMP is expected to ask the Senate to turn over all documents relating to per diems, gas mileage and other expenses claimed by the three senators.

Liberal senators will agree to hand over any information the RCMP requests and say they hope Conservative senators will, as well.

Sources say the RCMP is questioning the finding by Deloitte that some of the expense guidelines were unclear. The sources noted that all three senators declared as their primary residence a home in which they don’t live full-time.

“You know, ‘Where do you live?’ Liberal MP Rodger Cuzner said. “It’s not as easy as, ‘What is your name?’ But still it’s pretty straight-forward. So the fuzzy stuff doesn’t wash with Canadians.”

The Senate committee discussed whether to refer the matter to the RCMP, but in the end decided against it.

Conservative Senate Leader Marjory LeBreton said police were not called in because of the finding by Deloitte that some of the expense guidelines were unclear. She said that in response to that finding, the committee was recommending a number of “meaningful changes” to improve expense claim oversight.

“There’s no more of the honour system around this place,” LeBreton said.

Senators can claim an annual housing allowance of up to $22,000 if their primary residence is more than 100 kilometres outside of Ottawa.

Harb had claimed as his primary residence a bungalow in Westmeath, Ont., which is located more than 100 km northwest of Ottawa. However, neighbours told CTV earlier this year that no one seems to live in the house year-round. Harb has since put the home up for sale.

Using cellphone records and other investigative tools, the auditors determined that Harb spent about 21 per cent of his time at the bungalow over two years.

Brazeau's housing expense claims were audited because he claimed an address in Maniwaki, Que., as his primary residence, despite the fact that it is believed he lived in a home in Gatineau, a short drive from Parliament Hill.

Cellphone records show Brazeau spent only about 10 per cent of his time in Maniwaki.

On Friday, the opposition hammered the federal government over the issue during question period.

“If average Canadians did this, they would have to face real consequences under the law for fraud,” NDP MP Megan Leslie said in the House.

The government insisted that there is no evidence that any of the senators engaged in deliberate fraud, and declined to formally refer the matter to the RCMP.

“There is nothing in the audit to suggest that is the case here,” Government House Leader Peter Van Loan said. “What the audit found is there were inappropriate expenses and that the rules were unclear.”

Sources also said Friday that the Mounties are monitoring the ongoing audit into hundreds of thousands of dollars of air travel by Conservative Sen. Pamela Wallin.

In a statement to CTV News, the RCMP said Friday night that it would only acknowledge an investigation once criminal charges are laid.

With reports from CTV’s Ottawa Bureau Chief Robert Fife