TORONTO -- Ontario's opposition parties say a letter sent by Premier Kathleen Wynne to a Liberal riding association calls into question her timeline in a byelection scandal that has triggered a police investigation.

The Canadian Press has learned the premier provided written notice to the Sudbury Liberal riding association of her decision to appoint a candidate on Jan. 7 -- well after she says she personally made the decision.

Wynne has said that by the time two Liberals had conversations with "failed candidate" Andrew Olivier in December, she had already decided weeks earlier to appoint former New Democrat MP Glenn Thibeault as the Liberal candidate for the Feb. 5 byelection in Sudbury.

The Ontario Provincial Police are investigating allegations of bribery and corruption after Olivier said he was offered a job or appointment to step aside.

The Ontario Liberal Party constitution says the leader "shall" tell the nomination commissioner and riding association president in writing "as soon as possible" when the leader decides to appoint a candidate rather than hold a nomination.

Wynne would not answer NDP questions in the legislature today asking when she sent that letter, but The Canadian Press has learned it was sent on Jan. 7 -- the same day the premier called the byelection.

Bill Nurmi, the now-former Sudbury Liberal riding association president, says he received the letter via email on that date, weeks after he learned about Thibeault's appointment on the news, when it was announced publicly.

The Liberal riding association met Nov. 26 to discuss the resignation of NDP MPP Joe Cimino and agreed a nomination would be held, Nurmi said in an interview.

Wynne has said once she met Thibeault she "was convinced that he was the right candidate" and at that point decided to appoint him. The premier has only said that meeting took place at the end of November. Thibeault said Tuesday the two of them met Nov. 30.

The premier's office confirmed Tuesday afternoon that Wynne sent the letter confirming his appointment the day of the byelection call but did not address whether it was sent "as soon as possible," as per the party constitution.

NDP Leader Andrea Horwath said the Jan. 7 date of the letter calls into question the premier's statements that the Liberals were just trying to keep Olivier involved in the party in their discussions in December, and were not offering him anything to step aside.

Horwath said, "It's the smoking gun that shows that in fact Kathleen Wynne had not made a firm decision" when Liberals Gerry Lougheed and Pat Sorbara, Wynne's deputy chief of staff, spoke to Olivier.

Progressive Conservative Steve Clark said the dates "don't add up."

"If I was to dissect what the premier said in question period I would expect that letter would have been sent a lot earlier," he said.