Canadian Prime Minister Stephen Harper's right-hand man, Nigel Wright, resigned suddenly last month after revealing that he paid Can$90,000 (US$87,700) to Senator Mike Duffy in order to help the lawmaker repay funds he had wrongly claimed as Senate expenses.
After the repayment, Duffy stopped cooperating with an audit, leading to opposition cries of a cover-up and demands for a probe.
The Royal Canadian Mounted Police told AFP it was examining all information related to the transaction to "determine whether a criminal act has taken place."
"The RCMP would only lay charges when there is sufficient evidence to do so," spokeswoman Corporal Lucy Shorey said in a statement.
Duffy and two other Conservative senators facing spending scrutiny -- all appointed by Harper -- have since been tossed from the Tory caucus, but it was Wright's mea culpa that shocked Canadians as it marked the first scandal to reach into the prime minister's office since Harper was elected in 2006.
Federal Ethics Commissioner Mary Dawson leaked the RCMP investigation when she earlier announced the suspension of her own examination of the check written to Duffy by Harper's top aide.
Since Wright was also being investigated for the same matter by the RCMP to determine whether he has committed an offense, Dawson had to cease her review of whether Wright was in a conflict of interest when he wrote the check, she told public broadcaster CBC.
Harper has said Wright simply wrote a personal check to a friend in need, and had not consulted with him beforehand, though he also criticized Wright for what he called poor judgment.
"I'm not happy. I'm very upset about some conduct we have witnessed, the conduct of some parliamentarians and the conduct of my own office," Harper told his caucus last month.