The RCMP have zeroed in on some of suspended senator Mike Duffy's travels in the weeks after he'd been master of ceremonies at an event for Prime Minister Stephen Harper in June 2009, CBC News has learned.

The Mounties have charged Duffy with 31 counts of bribery, fraud and breach of trust in relation to his Senate expenses.

It is those allegations of fraud that brought the RCMP to speak to several current and former Conservative members of Parliament.

Sources tell CBC News former Conservative MP Dean Del Mastro has been asked to testify at the trial that begins April 7. Ontario MP Barry Devolin has also been served notice he may be a witness.

Former Conservative cabinet minister Gary Lunn has also spoken to the RCMP about Duffy.

The RCMP were trying to determine whether Duffy's work was for legitimate Senate business or, as the RCMP allege, for "personal and partisan reasons."

Travelled to MPs' events

On June 11, 2009, in Cambridge, Ont., the government spent tens of thousands of dollars to promote its plan to help rebuild the economy after the recession.

Duffy was there as master of ceremonies and to throw some friendly questions at the prime minister.

But the next day Duffy was back to more partisan work, attending a Conservative Party fundraiser in Belleville, Ont., for MP Daryl Kramp.

At the time, Duffy was still a relatively new senator and a hot ticket for the Conservatives. Duffy was a draw, and if the goal was raising money, MPs wanted him in the ridings.​

A week later, on June 19, 2009, Duffy was the guest speaker at another party fundraiser in Peterborough, Ont., for some 140 people.

Sources tell CBC News that Del Mastro has given authorities information about Duffy's involvement with that event.

The next day, on June 20, Duffy jumped one riding over to Devolin's constituency of Halburton-Kawartha Lakes, where he was described as the "special guest" at the fifth annual "Breakfast with Barry."

Devolin was interviewed by the RCMP this past December. The Mounties wanted to know who paid for Duffy's accommodations while he was in the riding and who drove him where.

A source with knowledge of the investigation said local supporters picked Duffy up on the Saturday morning and drove him to Toronto later that day. The riding association did not pay for any of Duffy's expenses and it was clearly a partisan event.

But the RCMP's interest in this particular gathering may relate to their belief Duffy improperly charged the expenses to the Senate.

Days later, Duffy was out in British Columbia attending more party fundraisers for other Conservative MPs.

Lunn, a former Conservative minister who used to represent the riding of Saanich-Gulf Islands, confirmed the RCMP have called him, but was reluctant to share more details with CBC News.

Lunn did confirm Duffy did not participate in events with him.

Duffy has always maintained he did nothing wrong.

The Senate rules from 2012 say senators can participate "in party activities that are related to the work of the senator or the Senate and its proceedings."

This definition of what is acceptable for senators may be a key issue in the trial that is expected to last 41 days.