Elections Canada's robocalls investigator says some evidence provided through Conservative Party lawyer Arthur Hamilton is "hearsay" and has to be backed up with witness testimony at trial.

Hamilton acted as a liaison between some witnesses and the election agency's investigators, and accompanied others to their interviews with investigators.

Michael Sona, 25, is the only person charged after an investigation into allegedly misleading phone calls in Guelph, Ont., in the 2011 federal election. Sona handled media relations for Conservative candidate Marty Burke. He is charged with wilfully preventing or trying to prevent a voter from casting a ballot. His next court date is June 2, 2014, which could be either a pre-trial or trial.

A short section of notes written by Elections Canada investigator Al Mathews and provided to CBC News shows that Mathews feels evidence provided through Hamilton is inadmissible unless it's confirmed through testimony in court.

The evidence comes from Andrew Prescott, Burke's deputy campaign manager. Elections Canada has traced the misleading robocalls to Burke's campaign office after a two-year investigation, which the agency says is continuing.

It's illegal to interfere with a voter's right to cast a ballot.

It's not clear who paid for Hamilton's service — some witnesses, who allegedly told Mathews that Sona bragged to them he'd planned and executed the misleading calls, have told CBC News that they are declining comment on the advice of their lawyer. But Hamilton told Elections Canada that he was acting on behalf of the party when he sat in on the interviews.

The names of those witnesses, as well as the number of witnesses and much of what they told Mathews, are in an affidavit he filed in August. The affidavit is covered under a publication ban that CBC News and other media organizations are fighting.

Witnesses refuse to talk

Prescott and Sona have refused to speak directly to Elections Canada investigators, Mathews wrote in affidavits filed in Ottawa court.

Mathews did get a copy of an invoice and expense list from Prescott, through Hamilton, as well as an email Prescott had sent to Sona and campaign manager Ken Morgan. The email contained the private phone number for RackNine, the company used to make the misleading robocalls, which Prescott had used before and was using for the Burke campaign. Other tools used to arrange the calls — an anonymous email address and a prepaid cellphone and credit cards — were procured over the ensuing four hours.

Mathews frames the email as a response, though he doesn't say whether he has an email from Morgan and Sona requesting the information.

Mathews wanted the expense list and invoice because RackNine, despite being used for the campaign in Guelph, didn't appear on a list of expenses for Burke's race for MP.

Mathews wrote in his notes on the investigation that Prescott would have to testify in court about the email or it would be considered hearsay. The same goes for the invoice and expense list that apparently cover Prescott's services during the campaign, as well as the explanation provided by Prescott through Hamilton for why no RackNine expense appears in campaign records.

"Should Prescott take the stand as a witness both the list and account can, of course, be put to him and adopted," Mathews wrote, noting that the same assessment applies to the email and the expenses list.

"If Prescott is an accused the account should be admissible as an admission."

Delayed investigation

Prescott and Sona aren't the only two who have declined to speak to Elections Canada. Mathews notes in a court document from May 25, 2012, that Prescott, Morgan, who moved to Kuwait, and another campaign volunteer, Trent Blanchette, have refused to speak to him.

It's something Chief Electoral Officer Marc Mayrand lamented in an interview with Evan Solomon, host of CBC Radio's The House.

Mayrand pointed out that elections investigators don't have the power to compel witnesses to talk and said some witnesses' unwillingness to speak to provide information has delayed the robocalls investigation in Guelph.

"It's a good example of a case where three people the investigator thought would have relevant information basically turned down any offer to be interviewed," Mayrand told Solomon.

"Sometimes, they [investigations] get into a dead end for lack of co-operation from witnesses," Mayrand said.