The federal grand jury investigation comes after a series of hearings at the D.C. Council probing whether Gray's 2010 mayoral campaign compensated former mayoral candidate Sulaimon Brown for his support of Gray during the campaign.
Gray has repeatedly denied any wrongdoing, and his attorney, Robert Bennett, tells NBC4's Tom Sherwood that Gray has not appeared before a grand jury.
Brown claims that Gray's campaign paid him to attack then Mayor Adrian Fenty, during the campaign. The quid-pro-quo scheme has also been connected to Brown's placement in a $110,000 per year position with the D.C. government -- from which he was fired in February.
When Brown testified before the council, he also made claims that Gray himself had knowledge of the payoff.
The grand jury investigation won't necessarily lead to an indictment, according to the NBC report.
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