An Elections Canada investigation prompted by misleading calls to voters during the final days of last May's election is spreading, CBC News has learned, as Prime Minister Stephen Harper says his party's national campaign had absolutely nothing to do with the calls.

The growing probe will involve interviews with some employees of a call centre based in Thunder Bay, Ont., operated by Responsive Marketing Group. The company was hired by the Conservatives to reach out to voters.

It was not immediately clear whether the interviews in Thunder Bay are directly connected with the continuing investigation by Elections Canada in Guelph, Ont.

A former employee of the RMG call centre in Thunder Bay told CBC earlier this week that some staff were poorly trained and that could be one reason voters may have been given false information about polling station locations.

Another RMG worker, who asked to remain anonymous, described to CBC Radio's As it Happens on Monday her discomfort with the way she felt her fellow employees were asked to influence voters on behalf of the Conservative Party. They weren't allowed to say they worked for a call centre, she said.

The employee said she and other workers took their concerns to the RCMP, but "they said they didn't want to listen, it wasn't their area, and it was now all over and done with."

'Smear campaign'

In question period Wednesday, the Conservatives changed tactics, accusing the Liberals and NDP of being sore losers.

Opposition MPs have a list of 45 ridings they say were targeted by automated robocalls or live calls wrongly telling voters their polling locations had changed, or harassing calls late at night or on religious holidays.

The controversy's epicentre is in Guelph, where Elections Canada is investigating allegations someone from the Conservative campaign deliberately tried to suppress votes by impersonating the election agency in robocalls directing people to the wrong voting location.

It's illegal to prevent a person from voting and to induce somebody to vote or not vote for a particular candidate.

Harper issued his strongest denial yet about whether his party had anything to do with the robocalls.

"The Conservative Party can say absolutely, definitively, it has no role in any of this," Harper said Wednesday in response to a question from NDP Leader Nycole Turmel.

"The honourable leader of the NDP should provide her party's information to Elections Canada," Harper said. "Otherwise, I think we just conclude this is simply a smear campaign without any basis at all."

A spokesman for Harper says he was referring to the Conservatives' national campaign, as opposed to individual local campaigns.

NDP MPs pointed out that only the Tories have had to pay a fine for breaking election laws. Charges against Doug Finley, who sits as a Conservative senator and other party officials were dropped as part of an agreement that saw the party pay $52,000 in fines for moving money from the national campaign to local campaigns and back in 2006, a tactic that became known as "in and out."

Conservative MPs had pointed to Guelph campaign worker Michael Sona, who stepped down from his job with MP Eve Adams last week, as being behind the robocalls. But in a statement Tuesday, Sona denied he had anything to do with it and said he resigned from his job because of the controversy over his involvement.

"I have remained silent to this point with the hope that the real guilty party would be apprehended," he said. "The rumours continue to swirl and media are now involving my family, so I feel that it is imperative that I respond."

Top Tory says story blown out of proportion

Earlier Wednesday, Finley, who ran the the Conservative Party's 2006 and 2008 election campaign, said misleading robocalls in Guelph are an isolated case and that the case of the strange calls has been blown out of proportion.

He also defended RMG, distinguishing its live call work from automated robocalls.

" The Conservative Party has been extraordinarily careful to ensure we use the best in the business. And the best in the business, in our view, is Responsive Marketing Group," Finley said.

Robocalling and live campaign calls are legitimate methods of campaigning used by all parties, he said. The calls cost pennies and some call centres in Canada are capable of making 200,000 to 300,000 calls an hour, he said.

"Quite frankly, this thing is blown out of proportion. That's No. 1," said Finley. "No. 2 … if there was a particular attempt at this voter suppression, in Guelph or anywhere else, I have no idea.

"This is the whole point, is that central campaign does not know because they had absolutely no idea what was happening."

The Conservatives' national campaign uses RMG, Finley said, not Racknine, the call centre through which the Guelph robocalls were placed.

Nine local Conservative campaigns, including Harper's, listed expenses for Racknine in their filings with Elections Canada for the May 2, 2011 election.

"There hasn’t [been] so far, as far as I can determine, one single issue of voter suppression — not one," Finley told CBC News. "To me it would appear it’s very isolated. If Guelph is where it is, it’s where it is."