Quebec's political class was left hanging in uncertainty and confusion Thursday after the arrest of a well-respected member of the National Assembly on suspicion he helped moles leak confidential police information.

Guy Ouellette, a 10-year MNA and former cop who was respected for his integrity on all sides of the legislature, was arrested on Wednesday afternoon by the province's anti-corruption unit and released early Thursday morning. Police have not charged him with any crime and have refused to offer any information on why they arrested an elected official – secrecy criticized by members from all parties.

Quebec's Unité permanante anti-corruption (UPAC) confirmed Thursday it conducted two raids on homes Wednesday and arrested one unnamed person on suspicion of abuse of confidence and obstruction of justice. Mr. Ouellette was identified as that suspect in the National Assembly Thursday as he withdrew temporarily from the Liberal caucus.

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The fog surrounding the case lifted slightly late Thursday – thanks to more leaks about the investigation, this time to the TVA television network. It reported UPAC was probing Mr. Ouellette's communications and contacts with police along with several people well known for exposing the province's corruption scandal, including former construction boss Lino Zambito and union whistle-blower Ken Pereira.

Police obtained Mr. Ouellette's phone records going back three years in a bid to establish he was acting as a go-between between police moles, journalists and whistle-blowers, the network said.

Still the basic question asked repeatedly across the province remained unanswered: Why would Mr. Ouellette, a member in good standing of the Liberal government, help leak information about a police investigation into Quebec Liberal Party fundraising and Jean Charest, the premier who recruited him for the 2007 election?

The UPAC statement said the arrest was necessary to secure evidence and to "prevent the infractions from continuing or repeating."

"Investigators will now analyze the evidence collected [Wednesday] before the case is submitted to prosecutors," the UPAC statement said.

Liberal caucus chair Filomena Rotiroti said she spoke with Mr. Ouellette late Wednesday night. "He simply said he was withdrawing from caucus. He was a bit shaken like all of us. Guy was always a team player so he decided it was the right decision while we wait for light to be shed," Ms. Rotiroti told reporters outside the caucus meeting.

Premier Philippe Couillard rushed back from a northern Quebec tour saying he wanted to help his team "absorb this news together and talk about it." The Premier said he was in the dark about allegations against Mr. Ouellette and he hoped to hear more soon.

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Mr. Ouellette was an investigator for the Sûreté du Quebec for 20 years and is the chair of a committee that oversees UPAC, but neither function explains how he might have access to confidential UPAC documents.

UPAC was launched in 2011 by the Charest government amid public outcry over the province's corruption scandal and suspicion other police forces had let bribery and graft go on.

Former senior Sûreté du Quebec officer Robert Lafrenière was named to lead the unit and it was embroiled in power struggles from the beginning. Mr. Lafrenière fired investigator and former Montreal police chief Jacques Duchesneau, who had a big hand in exposing corruption in the first place, over Mr. Duchesneau's vocal support for a public inquiry that was eventually held.

More recently Mr. Ouellette clashed with Mr. Lafrenière. Mr. Ouellette was reportedly opposed to the renewal of Mr. Lafrenière's contract in 2016. Mr. Ouellette was also opposed to a law making its way through the National Assembly that would give more autonomy, power and money to UPAC. The friction manifested itself last spring when the two got into a public spat over whether Mr. Lafrenière would appear before Mr. Ouellette's committee.

During a hearing last May, where Mr. Lafrenière was questioned about leaks in his unit, he looked directly at Mr. Ouellette and vowed to "find the bandit" who was spreading confidential information.

Many members of the National Assembly stood by Mr. Ouellette as a model of integrity on Thursday. Several also expressed concern about UPAC's handling of the case.

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"When they arrest someone, there has to be a reason. All we're left with is theories and [UPAC's] statement is not nearly detailed enough as far as I'm concerned," said Health Minister Gaétan Barrette.

Opponents were even more forceful rallying to Mr. Ouellette's defence.

"I want Guy Ouellette to know he has my respect and support," said Amir Khadir, a member of the left-wing Québec Solidaire Party. "From what I know of this man, I have no reason to doubt his integrity and probity."

The Parti Québécois public-security critic, Pascal Bérubé, said he was "dumbfounded."

"We need more information. We need to find out the truth," he said.

Mr. Ouellette, 65, who represents a riding in the Montreal suburb of Laval, became a household name in Quebec in the 1990s and early 2000s as a key investigator and, later, expert on the motorcycle gangs that were then terrorizing the province. He was recruited by Mr. Charest and first won his seat in 2007.

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The MNA was known as "Mr. Integrity" in the National Assembly – for a junior minister role he once played monitoring the integrity of the public administration, along with his law-enforcement past and public image.

However, there was occasionally friction within the Liberal Party. Last spring, he complained publicly that elements in his party were trying to push him out to make way for fresh candidates. Mr. Couillard had to reassure him that he was still a valued member of the government.