Two Canada Revenue Agency employees have been arrested and two others suspended for their alleged roles in connection with a $4.5-million tax-fraud scheme, Revenue Minister Jean-Pierre Blackburn said Wednesday.

Blackburn said the two arrested employees were team leaders at the Montreal tax-services office of the CRA.

In an interview with CTV Newsnet's Power Play, Blackburn said his organization called in the RCMP after the CRA's investigation was underway.

"It shows that our own internal investigation works quite well," he said. "Our employees are professional, there's 45,000 of them, but sometimes (these) situations happen."

At a news conference in Montreal earlier on Wednesday, he referred questions about possible charges about the two employees to the RCMP.

Blackburn said the agency's investigation was originally looking into firms owned by businessman Tony Accurso, a leader in the Montreal construction industry.

The CRA alleges that three construction companies, Simard-Beaudry Construction Inc., Construction Louisbourg Ltee., and Hyprescon, funnelled nearly $4.5 million into two dummy corporations that were not involved in any commercial business. The agency says the dummy corporations were used to falsify invoices.

The CRA says that in these types of schemes, the invoices are fraudulently used to reduce individuals' or companies' revenues for tax savings purposes.

The four CRA employees, who are all suspended without pay pending an internal agency investigation, allegedly tried to hide the activities from the tax agency.

The investigation was launched as a result of regular internal audits into the agency's workings, Blackburn said.

"The system is well-protected," he said.

But opposition critics says that the audit system needs to be tightened, considering that there are billions going to be spent in infrastructure as part of the government economic stimulus plans.

"It's very troubling (and) it's not very good timing for Canadians who are struggling and trying to make ends meet," Liberal treasury board critic Dan McTeague said.

None of the allegations have been proven in court.