Ian Rapoport of NFL Network received word from the league about the investigation, which is required by the collective bargaining agreement.
Williams held out through the first eight weeks of the season in hopes of being traded after JP Finlay of NBC Sports reported in August that he had "lost faith" with the medical staff.
"It was cancer. I had a tumor removed from my skull—attached to my skull—it got pretty serious for a second," Williams told reporters Tuesday. "I was told some scary things from the doctors. It was definitely nothing to play with. It was one of those things that will change your outlook on life."
Williams said there is "no trust" remaining between himself and the team. Washington failed to trade Williams before Tuesday's 4 p.m. ET deadline, at which point he reported to practice but has not participated due to discomfort with his helmet. He said he reported because he wants to accrue another year toward free agency but would not commit to playing in 2019.
"I mean, the lump continued to grow over the years. It was concerning, but there was no pain involved, and if I'm being told by the very people I put my career in the hands of, people are telling me I'm fine, I'm fine," Williams said. "That's how I looked at it."
According to Williams, he was diagnosed with dermatofibrosarcoma protuberans, a rare form of cancer. Doctors told him the cancer was removed weeks before it would have reached his skull, and Williams said he told his two daughters goodbye before going into surgery for the growth's removal.
Washington said it is planning an independent review into the matter.