Five former workers at a Rio Tinto Group aluminium smelter in Quebec were arrested following a fraud investigation initiated by the company almost two years ago.

The alleged fraud involved fake invoices from a former subcontractor of the company, according to a statement on Wednesday from Surete du Quebec, the police force in the Canadian province. The workers were allegedly paid bribes by the former subcontractor, police said. There is a warrant out for the arrest of a sixth former employee in the case.

Police have been investigating the matter since 2014, following an internal audit by Rio's Alcan unit, according to the statement.

"Rio Tinto is collaborating with the provincial authorities with regards to this investigation," the company said in an emailed statement to Bloomberg News. "We will refrain from commenting further as the investigation is ongoing."

Rio said in March 2015 it had filed a formal complaint for fraud to local police, who it said were probing companies with business ties to Rio's Alcan division in the Saguenay-Lac-Saint-Jean region.

Rio's Alma smelter began operating in 2000 and produced over 466,000 tons of primary aluminium last year.