RCMP investigators offered a detailed rationale for their decision not to charge Nigel Wright for giving Senator Mike Duffy $90,000 to pay off his expenses, documents show.

The documents, obtained by The Globe and Mail through a federal access-to-information request, go over the four charges the Mounties considered laying against Stephen Harper's former chief of staff before ultimately determining his value as a witness against Mr. Duffy was greater than the prospect of his conviction.

"The decision on whether or not to seek charges against Mr. Wright must be an assessment on the weight of value of the evidence he can provide in the matter of Senator Duffy, versus the prospects of a conviction on any charges that may be brought against him," former RCMP superintendent Biage Carrese wrote in his April, 2014 report, "Investigation of Nigel Wright Gifting 90K to Senator Duffy."

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"The primary focus of this investigation was on members [of] the Senate of Canada."

Related: RCMP passed on charging former Harper aide Nigel Wright to get his testimony in Duffy trial

Read more: The Mike Duffy dossier: What you missed in his courtroom drama

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

In his report, Mr. Carrese wrote that "there may be sufficient evidence to charge Nigel Wright" with frauds on the government.

"It could be argued that Mr. Wright was trying to 'pay off' Senator Duffy so that the matter would no longer be in the public domain and be an embarrassment to the government; or that the matter may have shown that Senator Duffy was not qualified under the Constitution to sit as a senator from PEI," Mr. Carrese wrote.

But when considering available evidence and without co-operation from Mr. Duffy or his lawyer, Janice Payne, investigators believed "proving all the elements of the offence may be difficult." He added the section of the law traditionally applied to an advantage or benefit with regard to contracting. He wrote that criminal intent "may be difficult to prove beyond a reasonable doubt."

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Likewise, for breach of trust, investigators had to prove Mr. Wright or someone else received a benefit in order to secure a conviction.

"The deal between Senator Duffy, and Nigel Wright and the PMO, may have provided a benefit to the Conservative Party," Mr. Carrese wrote.

"The matter of Senator Duffy's expenses was an embarrassment and distraction to the government, and it was on this basis presumably that Mr. Wright wanted the matter to [be] dealt with."

But the practice is to require a personal or "tangible benefit," Mr. Caresse wrote, and prosecutors had to prove "mens rea" – intention of wrongdoing.

"The evidence in fact shows that Mr. Wright thought it in the public interest to repay the $90K. He did not want the taxpayer to have to pay for Senator Duffy's expenses, which he believed the senator ought not have claimed on moral and ethical grounds," Mr. Carrese wrote.

In considering the bribery charge, Mr. Caresse said it was Mr. Duffy who "devised or proposed the scheme to be reimbursed for his ineligible expenses." At his trial, Mr. Duffy said he never did anything wrong and was following the Senate rules as they existed at the time. In his scathing decision, Ontario Justice Charles Vaillancourt condemned the "mind-boggling and shocking" conduct of the PMO and acquitted Mr. Duffy on 31 charges of fraud, breach of trust and bribery.

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"The evidence has shown that Mr. Wright was consistent in telling Senator Duffy to reimburse the Senate," Mr. Carrese wrote. "Had Senator Duffy done so without later making demands of the PMO and Nigel Wright, Mr. Wright would not have been placed in a position of gifting any monies. Mr. Wright's actions would have to be proven to be for a corrupt purpose."

Investigators felt the Parliament of Canada Act offence did not apply to Mr. Wright because Mr. Duffy did not render a "service" as part of his repayment deal.