While tourists were enjoying the splendours of Parliament Hill this summer, behind the scenes RCMP investigators were secretly hunkered down in a former Senate smoking room, interrogating witnesses in what has become the largest police probe involving parliamentarians in years.

CBC News has been given an exclusive glimpse inside the RCMP investigation of the Senate expenses scandal through interviews with a number of people close to the police probe. All spoke on condition of anonymity.

The investigation is being headed by Cpl. Greg Horton, a 21-year veteran of major crime investigations and now assigned to the special RCMP branch that deals with "sensitive" matters of "significant risk to Canada's political, economic and social integrity."

CBC sources were unanimous on one point: Horton and his investigators are tough, thorough, extremely well-prepared and seem deadly serious about a probe that could lead to criminal charges of fraud and breach of trust.

Sources say the federal Office of Public Prosecutions has already assigned a prosecutor to the Senate probe.

There is no indication the Mounties have interrogated any of the four senators actually at the centre of the expense scandal: Mike Duffy, Mac Harb, Patrick Brazeau and Pamela Wallin. Sources say investigators have not knocked on the prime minister's door either.

But CBC News has learned that over the past two months, Horton and his squad have questioned at least five other senators as key witnesses.

Some of those interviews were conducted in the sumptuous Salle de la Francophonie, an ornate meeting room near the main Senate chamber, decorated in paintings and sculptures of French kings, and used as a private smoking lounge for senators until the 1980s.

Although none of the senators questioned in the Francophonie this summer has been accused of any wrongdoing, sources say a Senate lawyer was usually present anyway.

Senator Mike Duffy leaves a meeting of the Senate internal economy committee on May 9 in Ottawa. An initial report on an audit of Duffy's expense claims was altered by a Senate subcommittee. (Sean Kilpatrick/Canadian Press)

So far, the sources say, the line of questioning has been focused in large part on Duffy's travel and housing claims, and on the now infamous $90,000 personal cheque he got from the prime minister's chief of staff, Nigel Wright, to reimburse the government for the disputed expenses.

They say the Mounties want to know what exactly was the deal Wright made with Duffy in return for the $90,000 cheque.

Who else in government and the Conservative Party knew about the deal? The RCMP say Wright has admitted telling three other high-ranking officials in the Prime Minister's Office, but not Stephen Harper.

Was there a secret deal in place with the Conservative Party — or some other source — to repay Wright the $90,000 he gave to Duffy?

For instance, sources say the Mounties have been asking specifically about Wright's sole access, as Harper's chief of staff, to a special stash of Conservative Party funds ostensibly to pay for partisan political activities in the Prime Minister's Office, a story first reported by CBC News.

Finally, sources say the Mounties are asking a lot of questions about how and why a Senate committee report on Duffy's housing and travel allowances was altered in the senator's favour, finding that he was merely the victim of errors and unclear rules.

As Horton says in one court filing: "I believe there was an agreement between Duffy and Wright involving repayment of the $90,000 and a Senate report that would not be critical of him, constituting an offence of frauds on the government."

CBC has confirmed that all three members of that Senate committee have been interviewed by the RCMP — Conservative senators David Tkachuk and Carolyn Stewart Olsen, Harper's former press secretary, and Liberal Senator George Furey.

Horton has accused Tkachuk of being in contact with both Duffy and Wright during the audit process, while chairing the committee that eventually watered down the audit report on Duffy’s expenses. Tkachuk claims he was never pressured to alter the report.

Marjory LeBreton, the Conservatives' former Senate leader, was also questioned by the Mounties this summer.

Finally, Cpl. Horton and his crew sat down with Senator Elizabeth Marshall, former auditor general of Newfoundland, and a member of a special Senate subcommittee that referred the expenses of Brazeau and Harb to the independent auditing firm Deloitte.

Until the RCMP have completed their investigation, it is impossible to predict whether criminal charges will be laid, and if so, against whom.

Most of those at the heart of the scandal have already moved to mitigate their respective situations.

Last week, Mac Harb dropped a lawsuit against the Senate over his expenses, agreed to pay back a total of $231,000, and resigned from the upper chamber, ensuring his parliamentary pension.

Senator Pamela Wallin speaks with Prime Minister Stephen Harper at a campaign rally during the 2011 federal election. A Senate report on Wallin's expenses has been forwarded to the RCMP, but the Mounties have not said whether they are investigating her. (Liam Richards/Canadian Press)

The Senate has referred an audit of Pamela Wallin's expenses to the RCMP, but the force has yet to indicate if it will investigate her.

No matter. She has already paid back $38,000 in expenses, and after an external audit, has agreed to refund the government an additional amount likely to top another $100,000.

Of course, Duffy has paid the government the $90,000 he got from Wright.

And both Duffy and Wright have hired some of the best criminal lawyers in the country.

So far, the RCMP have not indicated whether Wright is a witness or a suspect.