The D.C. Council committee investigating Mayor Vincent C. Gray’s hiring practices said Thursday that it has successfully served Sulaimon Brown with a subpoena requiring his testimony May 13.

Since Gray’s hiring practices came under scrutiny in February, the council has held three hearings examining who got jobs in the administration, how much senior staffers were being paid, and how some children of top staffers landed city jobs.

The Committee on Government Operations and the Environment, chaired by council member Mary M. Cheh (D-Ward 3), is also investigating whether there is a nexis between Brown being offered a $110,000 city job and allegations that he was paid by the two Gray campaign officials to attack former mayor Adrian M. Fenty (D) on the campaign trail.

Brown has refused to voluntarily testify, telling reporters he has instead decided to cooperate with an FBI investigation into the matter. The Washington Post has been unable to independently verify that Brown was paid.

In recent weeks, Cheh has expressed frustration that the committee had been unable to find Brown to serve him with a subpoena. The committee also reported two weeks that it was struggling to serve Cherita Whiting, a Gray friend who had been working at the Department of Parks and Recreation. Whiting resigned last month after news reports that she had a criminal record.

But in a statement, Cheh said Thursday that both Brown and Whiting “have been served.”

“The aim of this investigation is to lay out the facts about the executive’s personnel practices. The people of the District have a right to know if something improper happened, and if it did, we need to correct it,” Cheh said. “In order to lay out these facts and make a determination, we need to speak with these former hires.”

It remains unclear, however, whether Brown and Whiting will appear.

Brown was not immediately available to comment.

Reached by phone, Whiting denied that she has received a subpoena and said she has no plans to appear at the May 13 hearing.

“I have not been served anything,” Whiting said, adding that she was referring Cheh’s statement to her lawyer, A. Scott Bolden. “I have not been served and, secondly, I have nothing to say.”