WADA conducted an external independent review and discussed the report at meetings in Baku, Azerbaijan in November but determined the findings did not conclude the alleged bullying had taken place.
However, given the seriousness of the allegations, WADA decided a second investigation should be carried out and that the agency should seek legal advice.
In a statement WADA said the law firm Covington & Burling had "been given the mandate to conduct a full and thorough investigation of allegations of bullying and harassment, including interviews of the relevant parties".
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(Reporting by Steve Keating in Toronto. Editing by Martyn Herman)
