The RCMP shut down its three-year-long investigation into senator Pamela Wallin without laying any charges, less than one month after its case against senator Mike Duffy unravelled in court.

The national police force devoted massive resources to the two probes, raising a series of questions about the validity of the senators' expense claims in publicly available court documents.

Mr. Duffy was acquitted on 31 counts of fraud, breach of trust and bribery last month. Ms. Wallin expressed her relief on Thursday after learning she was no longer under investigation.

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"It has been a very long three years, and I'm glad this nightmare is over," Ms. Wallin told reporters as she entered the Senate.

Her lawyer, Terrence O'Sullivan, pointed out Ms. Wallin lost both of her parents during the investigation, and now has "extreme regrets they have not lived to see this day." He said he will soon discuss the possibility of seeking compensation from the Senate with Ms. Wallin, who was suspended for two years without pay.

"She was vilified in the press and in the public, walking through airports and restaurants with people making remarks about her," Mr. O'Sullivan said in an interview.

Having closed the Wallin file, the RCMP no longer has any active investigations into senators. However, a handful of cases that were red-flagged by the Auditor-General last year remain under review and could eventually become the subject of a formal investigation.

Senator Patrick Brazeau and retired senator Mac Harb are still awaiting their respective trials on charges of fraud and breach of trust, although it remains to be seen whether Mr. Duffy's acquittal will have an effect on the Crown's handling of those two cases.

The RCMP announced the end of the investigation into Ms. Wallin's expense claims with a terse statement on its website.

"Following consultation with Crown Counsel, the RCMP has determined that no criminal charges will be laid against Senator Wallin," Assistant Commissioner Gilles Michaud said in a news release.

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Ms. Wallin repaid a total of $154,191.29 in 2013 in expenses that external auditors found were either subject to reimbursement or subject to interpretation by the Senate's internal economy committee, including interest.

As part of a subsequent investigation, the RCMP alleged in court documents last year that it had identified 150 "suspicious" claims that were filed by Ms. Wallin.

After the RCMP announced the file was closed, NDP MP Charlie Angus said the Senate remains out of step with normal rules of accountability.

"This is a private club, these people can't be fired," he said. "When they break the rules with taxpayers' money, apparently there are no consequences."