GUELPH, Ont. -- Crown lawyers and Michael Sona's defence team agreed Monday it's likely multiple people were behind the thousands of misleading robocalls that went out to Liberal supporters on the morning of the 2011 federal election.

They also agree there are key questions about the fraud that may forever go unanswered -- and that one of the key witnesses who testified against Sona may have provided his trial with dubious testimony.

Both sides wrapped up their closing arguments Monday: the Crown argued Sona is clearly linked to the robocalls scandal, while the defence insisted the evidence doesn't definitively prove who was involved in the scheme.

On Aug. 14, some 39 months after misleading automated calls went out to 6,700 phone lines in Guelph, Ont., Justice Gary Hearn will rule on whether Sona is guilty of "wilfully preventing or endeavouring to prevent an elector from voting."

"Your Honour should be looking at proof of actions," said Sona's lawyer Norm Boxall, who elected to call no witnesses after the Crown introduced its evidence.

Boxall told court his client isn't required by law to prove his innocence, and said the Crown failed to make its case beyond a reasonable doubt.

"I think there's more unanswered questions than answered questions."

A mysterious figure used the fake names Pierre Poutine and Pierre Jones, along with untraceable prepaid credit cards and a disposable cellphone, to order an automated telephone campaign with the company RackNine Inc., court heard.

Sona lacked the technical savvy to orchestrate the complicated plot, said Boxall, arguing Andrew Prescott -- a colleague of Sona's who had experience with ordering robocalls -- is a more likely suspect.

Prescott testified against Sona last week in exchange for immunity, but even the Crown acknowledged it doesn't fully accept his evidence, Boxall said.

During his own closing arguments, Crown attorney Croft Michaelson's told Hearn more than once that Prescott's testimony "should probably be approached with caution."

"In my submission, Mr. Prescott is just not credible," said Boxall, who noted that his answers to questions changed over time and he was not fully forthcoming with the Crown until he secured an immunity deal.

"Mr. Prescott is deflecting responsibility from himself and perhaps others."

In his testimony, Prescott recalled seeing a cellphone on Sona's desk that resembled the type used to order the robocalls, Boxall noted -- an observation he never mentioned to investigators.

Boxall also questioned another key anecdote in which Prescott said Sona emerged from his cubicle on the morning of the election, saying excitedly, "It's working."

Prescott was the only one who testified to seeing Sona in the office that morning, Boxall said. Nor could anyone else corroborate Prescott's claim that Sona gave "thanks to Pierre" after the Conservatives won a majority government.

"The whole circumstances are very odd, we haven't heard any other witness describe it," Boxall said. "(The Crown case) is going to depend on Mr. Prescott's credibility, which is sadly lacking."

Hearn asked Michaelson if he considered the toast "damning evidence," to which he replied: "it's evidence."

Both the Crown and defence said they believe more than one person was involved, based on agreed-upon evidence in the case.

One of the prepaid credit cards used to order the robocalls was purchased at a Shoppers Drug Mart at the very same time that the RackNine account linked to the case was being accessed on a computer.

Boxall has alluded to another Conservative campaign worker as a possible key figure in the case.

Under cross-examination, Prescott spoke about Kenneth Morgan, the campaign manager for the local Conservative candidate Marty Burke. Prescott said he was ordered on election day by Morgan to log on to a RackNine account he did not recognize and "stop the calls."'

Morgan moved to Kuwait shortly after the 2011 campaign and, according to court documents, has refused to speak to Elections Canada investigators about the robocall affair. He has not been charged with any offence.

During his closing, Michaelson said the evidence in the case "points to more than one person" being involved, including Sona.

"Sona played an instrumental role and committed one or more acts" in the scheme, he said. There's more than enough evidence showing that Sona aided and abetted someone else, Michaelson argued.

"The evidence looked at as a whole should leave you with no reasonable doubt," he said. "Mr. Sona had the opportunity, the motive and the means to commit (the crime)."

Boxall said the testimony from witnesses who claimed they heard Sona confess was not strong enough to warrant a conviction.

Some of the facts Sona allegedly detailed in his stories didn't match up with evidence in the case, he added, suggesting Sona told tall tales to sound "more important than he is."