WASHINGTON (AP) – President Donald Trump is reviving concerns that the special counsel team investigating Russian election interference may be biased against him.

Trump tweeted Sunday: “Why does the Mueller team have 13 hardened Democrats, some big Crooked Hillary supporters, and Zero Republicans? Another Dem recently added…does anyone think this is fair? And yet, there is NO COLLUSION!”

The tweet marks one of the first times that Trump has mentioned special counsel Robert Mueller by name.

Trump allies previously have questioned whether Mueller can lead an unbiased probe because some of his investigators have made campaign contributions to Democratic candidates, including Hillary Clinton. Mueller is a former FBI director appointed by Republican President George W. Bush.

Other Trump backers have noted that Mueller interviewed with Trump to serve another term as FBI director. Trump instead nominated Christopher Wray to lead the bureau.