The RCMP had enough evidence to charge Nigel Wright but chose not to proceed so that the former chief of staff could testify in the criminal trial of Senator Mike Duffy, according to documents obtained by The Globe and Mail.

With an uncertain prospect of conviction, investigators chose a path that would allow Mr. Wright to remain an "unindicted co-conspirator," because they deemed Mr. Duffy, a sitting senator, to be of greater public interest than an unelected senior official in the Prime Minister's Office.

The documents, obtained through a federal access-to-information request, shed light on one of the enduring mysteries of the bribery probe that exposed the inner workings of former prime minister Stephen Harper's office: why the RCMP never charged Mr. Wright for his role in giving the senator more than $90,000 to pay off his expenses.

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"While sufficient evidence does exist to support charges against Mr. Wright, the prospect of obtaining convictions relating to those offences could be limited due to the statutory requirements," former RCMP superintendent Biage Carrese wrote in the conclusion to his April 14, 2014 report, "Investigation of Nigel Wright Gifting $90K to Senator Duffy."

"The decision by the investigative team is that it is in the public interest to secure convictions on the charges against Senator Duffy."

Sergeant Greg Horton, the lead investigator on the case, wrote in a March 19, 2014 report that any decision to charge Mr. Wright "must be weighed in the interest of the investigation itself, as well as what would serve the public best, irrespective of any criticism that may be levelled as a result of that decision."

"Charges against Mr. Wright could in fact inadvertently weaken a case against Senator Duffy, who is the primary focus of this investigation," Mr. Horton wrote.

"It would serve the public interest best for Mr. Wright to remain an unindicted co-conspirator, compelled to provide testimony against Senator Duffy relating to the material that he turned over to investigators."

The documents detail how investigators felt they needed Mr. Wright as a witness to explain the contents of two binders of e-mails from the Prime Minister's Office and Mr. Duffy's day calendar, which Mr. Wright had provided to the RCMP. Investigators proposed that the senator face only nine charges, although he was later charged with 31 offences.

"… The importance of the evidence that can be obtained from Nigel Wright as a witness cannot be overstated," Mr. Carrese wrote. The next day, the Mounties announced the evidence gathered "does not support criminal charges" against Mr. Wright.

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But it was those same e-mails and Mr. Wright's testimony about the lengths to which Mr. Harper's PMO tried to contain the controversy surrounding Mr. Duffy's expenses that played a key part in the senator's acquittal on charges of fraud, breach of trust and bribery.

In his scathing April 22 decision, Ontario Justice Charles Vaillancourt condemned "the mind-boggling and shocking conduct" of the PMO, which exerted what he called "threats and pressure" to get the senator to repay his expenses.

Mr. Wright's lawyer, Peter Mantas, said in a statement Friday that the RCMP's "detailed and thorough investigation" concluded there was no reasonable prospect of convicting Mr. Wright.

"In addition, Mr. Wright did not ask for favourable consideration from the RCMP, nor was it offered. The report underlines that Mr. Wright's intention was to secure the repayment of taxpayer funds and that he believed his actions were always in the public interest and lawful," Mr. Mantas said.

Mr. Horton described Mr. Wright as "co-operative and forthright" and said he provided "tangible evidence" that helped advance Mr. Duffy's investigation. He also wrote that Mr. Wright's "position on the government hierarchy was considerably lower than that of any senator or member of the House of Commons."

"The focus of this investigation is on members of the Senate of Canada, an institution at the top of our democratic government hierarchy, responsible for passing the laws which govern our country," Mr. Horton wrote.

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The report shows the Mounties considered laying four charges against Mr. Wright: breach of trust, frauds on the government, bribery of a judicial officer and receiving compensation as prohibited under the Parliament of Canada Act.

The Mounties were investigating Mr. Wright both for giving Mr. Duffy $90,000 but also for "exerting pressure" on a Senate sub-committee to "cleanse" a report relating to Mr. Duffy's audit, the document says.

Investigators also outlined their decision not to interview Mr. Harper as part of their investigation. Mr. Wright told the RCMP the former prime minister was not aware of his decision to personally cover the cost of Mr. Duffy's expenses.

"Evidence was gathered that Prime Minister Harper was aware in general terms only that Mr. Wright and PMO staffers were dealing with the matter of Senator Duffy, but no information or evidence was gathered that he knew specifically about the $90K," Mr. Carrese wrote.

"Based on this, there is no reason to believe that interviewing the Prime Minister would provide any additional evidence."