A criminal defense attorney representing a top Boston City Hall aide in an extortion case asked federal prosecutors on Monday for evidence that could cast doubt on a statement made by a former client.

Tom Kiley, whose lengthy client list has included a former House speaker, is representing Tim Sullivan, a former labor lobbyist accused of extorting a music festival to hire union stage hands in his role as City Hall's chief of intergovernmental relations.

Sullivan and City Hall tourism chief Ken Brissette are accused of withholding permits from the Boston Calling music festival, pressuring the group to hire members of the International Alliance of Theatrical Stage Employees (IATSE) Local 11.

Boston Mayor Marty Walsh has said both men are on paid leave and that he never directed employees to consider whether organized labor would receive jobs in permitting.

In a discovery request Monday, Kiley recounted a conference call between "the government" and a former client. The client is unnamed in the filing and Kiley declined to name the person in an interview with the News Service.

In the phone call, the unnamed government official asked Kiley's former client "whether he had warned Tim Sullivan about the legality of withholding, revoking or linking, in any way, the issuance of City of Boston Permits to conduct concerts on City Hall Plaza to the agreement to employ Local 11 members," according to Monday's discovery filing. The letter does not say how the former client responded.

According to the indictments against Brissette and Sullivan, in 2014 Boston's chief of operations - who was then Joe Rull - told Brissette that it was "not legal" for him to pull permits for Top Chef, a reality cooking show that was also allegedly pressured to hire union labor. The director of the State Film Office, a position held by Lisa Strout since 2011, made a similar warning to Brissette. The indictment also refers to warnings from "government officials" that Sullivan and Brissette allegedly did not heed when they pressured Boston Calling to hire union stagehands.

"To the extent my former client's grand jury testimony, exhibits or reports of any statements refutes or cast doubt on the veracity of his pre-indictment statement during the conference call described above, please provide that statement," Kiley wrote in his request for information from the government.

The conference call took place the evening before the grand jury returned Sullivan's indictment. The indictment was signed Tuesday, June 28, just after noon. That same day federal prosecutors secretly sought to remove Kiley from the case. The following morning Sullivan was arrested.

The prosecution's effort to disqualify Kiley was revealed by a motion Kiley made in court in late July.

Martin Healy, chief legal counsel for the Massachusetts Bar Association, said prosecutors can seek to disqualify an attorney if there is "an actual or perceived conflict of interest," such as a former client who is tied in to the current case.

"Clearly there are a number of intersecting witnesses that overlap in the case," Healy said. He said, "Tom Kiley has been the go-to lawyer for many individuals facing prosecutions of alleged misconduct involving both state and federal ethics laws."

Healy said judges give attempts to disqualify an attorney "very close scrutiny" because "they can be very disruptive to the person's current client," and said Kiley has "a professional duty to appropriately assess his ethical responsibilities regarding potential conflicts involving clients that he has represented and anyone he is now representing in the case at hand."

"It really can be detrimental to the current situation," said Healy, who said disqualifications can incur costs on the defendant and slow the case down. He said, "It would have to rise to a pretty high level of conflict for the judge to grant the motion to disqualify."

A spokeswoman for U.S. Attorney Carmen Ortiz said she could not comment, and Kiley also said he could not comment on a matter under seal.

"It's unfortunate that it's all being done under closed orders," Healy said.

Kiley's discovery letter includes a request to learn whether the Top Chef matter will be brought into the case against Sullivan and Brissette.

The show moved its filming out of Boston after Brissette spread word that it wasn't using union labor. Separately, federal prosecutors charged members of the Teamsters Local 25 with extortion for disrupting Top Chef filming in Milton.

As Kiley pointed out in a brief phone interview, despite "numerous paragraphs" devoted to Top Chef, the indictment of Brissette and Sullivan "charges nothing with respect to it."

Kiley asked prosecutors to disclose whether they plan to use any search warrant records related to Top Chef in Sullivan's case. Kiley's letter also indicates that prosecutors seized records from Google on March 10, 2015 - less than a year after Boston Calling hired union stagehands ahead of a September festival and a little over a year before Brissette was charged and arrested.

Sullivan's defense attorney also asked federal prosecutors to identify by name any known, putative, unindicted co-conspirators. The identities of unindicted co-conspirators - a designation assigned by the prosecution - has generated attention in prior white-collar crime cases as associates of the defendants, like the defendants themselves, are often well known in political, business and media circles.