Members of the committee that polices Senate expenses have called in two senators for private interviews, raising to six the number of senators whose spending of public funds has come into question this year.
The interviews, which began Monday afternoon, are being conducted by members of the Senate sub-committee on internal economy, according to a source familiar with the committee's work.
The two senators are being interviewed after the internal economy committee asked members to produce documents showing where they live.
The interviews do not by themselves indicate wrongdoing, but if the sub-committee is not convinced that the claims are legitimate, it can refer them to an external auditor.
The interview process began as the Conservative government argued in the House of Commons against allegations that some senators may have used taxpayer money improperly.
Conservative Senator Mike Duffy, independent Senator Patrick Brazeau and Liberal Senator Mac Harb are already being audited to determine where their primary residence is and whether they are entitled to collect a housing allowance.
Questions have also been raised about Conservative Senator Pamela Wallin's spending on travel between Ottawa and Toronto, where she owns a condominium. Ms. Wallin also owns a residence in Saskatchewan, the province she represents in the Senate.
Prime Minister Stephen Harper, who has appointed a majority of the members in the 105-seat chamber, must deal with allegations that some of those he appointed have acted improperly.
Late last year, the committee asked all senators to provide copies of their health cards, driver's licences, voting records and portions of their tax filings to show that they live where they say they do.
A subsequent letter from Marjory LeBreton, the Government Leader in the Senate, and James Cowan, the Opposition Leader in the Senate, asked the internal economy committee to grill anyone who did not or could not provide that information to determine whether they should pay back the money they claimed.
"We request that you proceed to interview each Senator who has claimed a secondary residence allowance to confirm the legitimacy of such claims," the letter said.
The first interview took place on Monday and the second is expected to be completed before the end of this week. The interviews are being conducted by the steering committee for the internal economy committee, which is made up of Conservative senators David Tkachuk and Carolyn Stewart Olsen and Liberal Senator George Furey. Senator Percy Downe filled in for Mr. Furey on Monday.
Mr. Cowan said he is aware that the meetings are taking place this week but does not know who is being interviewed.
"If there are other cases that arise as a result of the interview process that Senator LeBreton and I called for, presumably those will be referred to external auditors and the audit reports will be made public. And then if there's some further action that's required, that will be taken," he said.
He said the current process is proof that "the system has worked" and any inappropriate expenses will be dealt with.
Mr. Duffy announced late last week that he would repay the money he received for his housing allowance, saying he "may have been mistaken" when filling out a form about his primary residence.
The Harper government signalled its support for Mr. Duffy and, on Monday, backed all three Conservative senators whose expenses are being questioned.
Senator Dennis Patterson, who represents Nunavut and reportedly owns residences in both Vancouver and Iqaluit, was added to the list. It has not been made public whether Mr. Patterson is one of the two senators being interviewed this week.
"Senators Patterson, Wallin and Duffy all own property in the provinces and territory they represent," Government House Leader Peter Van Loan said in the House of Commons. "They maintain deep continuing ties to those regions."
NDP Leader Thomas Mulcair said he is not confident that the Senate will be able to appropriately discipline any senators who are found to have inappropriately billed taxpayers for their living or travel expenses.
"There's been no indication of their ability to do that in the past," he said. The NDP has said it would abolish the Senate.