County Manager David Smith and Assistant County Manager Sandi Wilson said they were preparing to testify before the grand jury on Wednesday.
Sheriff Joe Arpaio denied any knowledge of the grand jury, but news of sheriff's officials being called to testify before a federal panel has been circulating in county circles for months.
Republic sources have confirmed that at least three high-ranking sheriff's officials, including a captain and two chief deputies, have testified before a federal grand jury in the last four months, though the nature of their testimony remained unclear.
"I'm not going to comment on that situation," Arpaio said Thursday. "We're just going to continue doing our job and our investigations that we have in progress."
Sheriff's detectives have undertaken a series of controversial investigations into county officials in the last year which heated up in recent months with indictments of two members of the Board of Supervisors, and a wide-ranging racketeering complaint filed in federal court that alleges a vast conspiracy against county supervisors, judges, private attorney and administrators including Smith and Wilson.
But the allegations of abuse of power have dogged Arpaio since long before that claim was filed.
Public officials confirmed to the Republic in May that FBI agents were interviewing them about whether Arpaio had abused his authority.
Five public officials involved in ongoing disputes with the Sheriff's Office confirmed federal agents asked them questions that focus on one theme: possible misuse of power by Arpaio and other sheriff's representatives, perhaps related to the ongoing disputes between the sheriff, county supervisors and top county administrators.
As is customary, the FBI would not confirm the nature of its inquiry or even that one was taking place.
"They asked if we felt he (Arpaio) was intimidating us," said one source, referring to the FBI's line of questioning. "We talked a lot about the fight between the (Board of) Supervisors, (County Attorney Andrew) Thomas and the sheriff."
At the time, Arpaio said the claims were outrageous.
On Thursday night, he would only repeat his refusals to discuss anything related to a grand jury.
"I have no idea what's going on in the grand jury," he said. "I am not going to comment about grand jury activity- no matter what you hear."
A Department of Justice spokesman in Washington, D.C., said: "We have no comment as grand jury proceedings are secret and we can neither confirm or deny their existence."
A spokeswoman for the U.S. Attorney's Office in Phoenix would not comment. The department defines a grand jury as citizens who listen to evidence of criminal allegations, which prosecutors from the U.S. Attorney's Office present, in order to determine whether there is probable cause to believe a crime was committed.
A grand jury indictment would not be evidence of guilt, a factor determined in court.