The Hellman Commission was a committee established by the State of New York to seek out and expose corrupt police officers within the NYPD. Its five members were appointed by the Governor of New York, including the chairman, Judge Lawrence Hellman.
The Commission had subpoena power to compel appearance before it, and testimony was given under oath. It could make deals with those testifying about their own corrupt activities to get them to expose other corruption.
Detective John Flynn was brought before the Commission to testify about his activities, including how he murdered Richie Morales. He was given a generous deal for doing so, without jail time (though he would lose his pension). This was done to get Flynn to talk about other corruption in the Department, including an allegation against Lennie Briscoe later found to be without merit. The generous deal outraged EADA Jack McCoy, who wanted to get Flynn to give up the man he was working for, Garcia. (L&O: "Corruption")