Gerald Tremblay, who stepped down last November, has been perhaps the highest-ranking political casualty resulting from the probe.
The ex-mayor will appear as early as Wednesday after the latest witness, his former right-hand man Frank Zampino, completes his turn on the stand.
When he resigned last fall, Tremblay said he had hoped to testify while still in office so that he might have a chance to defend himself.
But he said inquiry officials had their own schedule and they wouldn't let him appear earlier so, realizing the extent of his political predicament, he had no choice but to step down.
The mayor had wanted to respond to allegations from a former aide that he knew about illegal financing in his political party — and that he ignored it.
That allegation was politically devastating to Tremblay, after he had spent years repeatedly proclaiming his ignorance of such wrongdoing.
But there has since been a new wrinkle.
The former aide whose testimony so damaged Tremblay, Martin Dumont, has more recently admitted that he made up details in other parts of his testimony.
As a result, the mayor's accuser has come under considerable scrutiny. Dumont has worked in a number of political roles — at city hall, for Tremblay's Union Montreal party, and in Ottawa for various offices in the Harper government including the Prime Minister's Office.
The ex-mayor's appearance will come after the grilling of his former confidant, Zampino, who as chairman of the municipal executive body was the No. 2 at city hall.
Zampino has raised eyebrows by contradicting multiple details from previous witnesses' testimony. On Tuesday, he even contradicted himself.
He had spent days on the stand denying any knowledge whatsoever of collusion in the awarding of local construction contracts. He changed his story after being confronted with some evidence Tuesday about a 2006 internal report that described collusion.
Zampino responded by saying he had sought to fight corrupt practices at the time. That claim drew a frustrated reaction from an inquiry commissioner.
"You lied before!" said commissioner Renaud Lachance.
"I asked you, 'Had you heard about signs of collusion in Montreal?' You said, 'No, not until (this) commission.' And now you're telling me you heard about it so clearly that you took a number of measures to try responding to it?"
When Lachance asked if he'd spoken with the mayor about the 2006 report, Zampino said he didn't recall any such conversation. This time, the inquiry chair stepped in.
"You, Mr. Zampino, were the mayor's right-hand man," France Charbonneau said.
"It was your responsibility to inform the mayor."
In separate criminal proceedings, Zampino faces a number of criminal charges — including fraud, conspiracy and breach of trust stemming from a City of Montreal land deal.
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