MONTREAL -- Quebec's corruption inquiry is set to resume Tuesday after a two-month-long summer break -- and this time it appears unions will be put under the microscope.

The probe has already rocked the province, uncovering everything from illegal political party financing to engineering firms colluding on contracts.

So far it has heard from 80 witnesses, including mayors, city bureaucrats, and engineering executives.

Now, six members of the powerful FTQ construction union, which represents 70,000 workers, are expected to testify at the inquiry this fall, including top boss Yves Ouellet.

Ouellet confirmed he had received notice to testify this fall, at some point between early September and the beginning of December.

"We've said all along that we would respond to their questions and that we would participate, so that's what well do," Ouellet said Friday in an interview with The Canadian Press.

"We'll be waiting for their call."

Other construction unions contacted by The Canadian Press said they had not been given notice to appear before the commission this fall.

It appears, as well, that the inquiry will finally turn its attention to the labour-sponsored fund Fonds de solidarite FTQ.

Questions from the commission are likely to bring unwanted attention to the investment fund, which is already dealing with a political problem. Federal Finance Minister Jim Flaherty eliminated some of its tax advantages in last year's budget.

Patrick McQuilken, a spokesman for Fonds de solidarite FTQ, wouldn't say whether anyone at the fund had been summoned for questioning.

"It's not for us to say who has been invited by the Charbonneau commission," he said.

During the fall session, the commission is also expected to delve into construction contracts involving Quebec's transportation ministry as well as cities around the province.

Montreal and Laval have already come under close scrutiny, bringing down mayors in both cities.

Municipalities like Gatineau and Quebec City are rumoured to be next -- something Richard Bourdon, a spokesman for the commission, would neither confirm nor deny.

Either way, the commission still has questions about Montreal and Laval, and has yet to closely examine the financing of provincial political parties.

The influence of organized crime in the construction industry is also due for a more in-depth look.

Chair France Charbonneau has been given lots of time to complete the probe.

The corruption inquiry got an 18-month extension from the provincial government.

The inquiry will be required to submit a progress report by Jan. 31, 2014.

Charbonneau must deliver her final report by April 2015.