The D.C. Council voted unanimously Tuesday to ask a judge to order three key figures to testify in its investigation of Mayor Vincent C. Gray’s hiring practices.

Sulaimon Brown, the central figure in allegations of Gray campaign payoffs, has refused to testify, questioning the motivations of the council’s probe, led by member Mary M. Cheh (D-Ward 3).

Cheh hired process servers to summon Brown and Cherita Whiting, a Gray supporter who was given a job with the city parks department, to appear at a May 13 hearing, but neither was successfully served in person. Cheh then sent the subpoenas by certified mail, as is permitted under council rules. But neither showed up at the hearing last Friday, where Cheh said that they “appear to have been evading service.”

Both Brown and Whiting have acknowledged in interviews that they are not cooperating with the investigation, which has been widely publicized.

The council also voted to force the testimony of Peyton M. Brooks, son of Howard Brooks, who is alleged to have delivered cash payments to Brown. (The Washington Post has been unable to independently verify those payments.)

Cheh asked both father and son to testify, but both declined, citing their constitutional right against self-incrimination. Cheh excused the senior Brooks, but she is questioning whether the younger Brooks is entitled to cite Fifth Amendment protections.

The resolution, which passed by voice vote with little debate, said that Peyton Brooks “has information directly relating to the subject matter of this investigation ... that is likely not protected by the Fifth Amendment privilege against self-incrimination.”

Council lawyers in the coming days will ask a D.C. Superior Court judge to order the three to appear before the council. If orders are granted, and if the witnesses flout them, they could be found in contempt of court and face criminal proceedings.