Two former top aides to John J. O’Brien when he was probation commissioner have been granted immunity from prosecution in ­exchange for testifying against him in federal and state corruption inves­tigations, a state prosecutor disclosed in court on Tuesday.

Prosecutors for Attorney General Martha Coakley confirmed Tuesday that Edward P. Ryan, who was O’Brien’s legislative liaison, as well as former deputy commissioner Francis M. Wall, are cooperating with the prosecutors. The two men can give prosecutors a firsthand view of the way O’Brien allegedly funneled jobs and promotions to polit­ically connected job candidates.

The two are among a growing number of probation officials and others who have agreed to cooperate with the sweeping investigation. People who have followed the 20-month probe closely say prosecutors have also granted immunity to ­numerous other witnesses before the state and federal grand juries and may be considering more.

The immunity deal-making, usually conducted in secret, is part of prosecutors’ strategy to build the strongest case against major figures in an agency that US Attorney ­Carmen M. Ortiz alleged was operated as a criminal enterprise.

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“The government does not usually enter into cooperation agreements without some expec­tation as to the assistance it will get. It is likely that the prosecutors expect to either strengthen existing ­cases or get information that will lead to new ones,” said Donald K. Stern, who served as US Attorney from 1993 to 2001.

Paul F. Ware, the independent counsel who alleged pervasive fraud in the Probation Department’s hiring practices in 2010, has said he suspects that prosecutors are looking to make a deal with O’Brien himself in exchange for testimony against politicians who benefited from the system.

Wall and Ryan are believed to have been cooperating with prosecutors for months — ­Ryan has testified at least three times already before grand juries — but confirmation came Tuesday when their names surfaced in Suffolk ­Superior Court about the state’s bribery case against O’Brien. A state grand jury charged that O’Brien organized a June 2005 fund-raiser for Timothy P. Cahill, then state treasurer, to help his wife, Laurie O’Brien, get a job at the state lottery, which was under Cahill’s control.

Cahill, who stepped down as treasurer to run unsuccessfully for governor in 2010, is not charged in the case. But Cahill’s former top aide and campaign manager, Scott Campbell, faces charges of conspiracy and state campaign finance violations.

Cahill’s attorney Joseph ­Demeo has insisted that there was no connection between the fund-raiser and the job for Laurie O’Brien.

At Tuesday’s hearing, Campbell’s lawyer, Charles Rankin, asked a judge to order prosecutors to explain what promises they had made to Ryan and Wall in exchange for their testimony. Rankin said that Ryan’s testimony before the grand jury since he ­received an immunity deal contradicts what he told Ware in his 2010 investigation. The change involved whether the fund-raiser for Cahill had some bearing on Laurie O’Brien’s hiring; Ryan first said it did not, but later said it did, according to Rankin.

Judge Bonnie H. MacLeod did not rule Tuesday on Rankin’s request for a full expla­nation of the deal with Ryan and Wall.

Assistant Attorney General Peter Mullin said in court documents that he expects Ryan and Wall to testify at O’Brien’s trial, scheduled for Sept. 24, as immunized witnesses, meaning they cannot be prosecuted for any wrongdoing in the case. Mullin wrote that ­Ryan could be especially valuable because he “played a central role in the 2005 Cahill fund-raiser and the hiring of Commissioner O’Brien’s wife at the lottery.”

The state charges carry much smaller penalties, a maximum of one to five years in prison per count, than the federal indictments handed up in March against O’Brien and two former deputies, ­William H. Burke III and ­Elizabeth V. Tavares. O’Brien faces federal racketeering, conspiracy and mail fraud charges that could result in up to 20 years in prison and $250,000 fines for his role in allegedly administering a rigged hiring process.

On Sunday, the Globe ­reported that since O’Brien, Tavares, and Burke were ­indicted, federal prosecutors have continued to call witnesses to the grand jury, focusing on Democratic legislators who benefited from the probation’s hiring system. The Globe Spotlight Team identified 250 probation employees who have personal or political connections to politicians or court officials.

Several lawmakers testified before the secret grand jury including Representative John Rogers, the Norwood representative whom DeLeo defeated to become speaker in 2009. Backers of Rogers have suspected that O’Brien helped DeLeo win the speakership by giving jobs to people close to legislators who agreed to back DeLeo, a charge both men ­deny.

Ware, a former federal prosecutor, said he had no ­direct knowledge, but he said it makes sense for prosecutors to negotiate a deal with O’Brien to get his cooperation in the investigation of politicians’ potential culpability.

O’Brien’s lawyers would not comment on either the federal or the state cases against their client.

Tavares’s lawyer, Brad ­Bailey, said that his client is not cooperating. “Our client continues to maintain her ­innocence and is looking forward to trial,” he said.

Court documents filed in the state bribery case over the last three months make clear the evolving nature of the inves­tigations. Assistant Attorney General Mullin said the state grand jury initially ­indicted O’Brien in February 2011 on a different charge, filing a false report in connection with the promotion of an assistant chief probation officer in the Worcester District Court, Bernard Dow.

Dow admitted that he failed to win promotions for almost 30 years until he started donating $500 a year to the campaign of former speaker Salvatore F. DiMasi.

“I knew I was not going to get that job on my qualifications alone,” Dow told Ware. “I knew or I believed that I needed some political help to get it.”

The grand jury indicted O’Brien for asserting that Dow had been the most qualified choice, but then sealed the indict­ment, making no public announcement, while the grand jury continued to hear testimony, Mullin wrote.

Then, the grand jury’s ­focus shifted to the Cahill fund-raiser. Between April and June, 60 witnesses testified about the fund-raiser. On June 20, three days before a statute of limitation would ­expire and make it impossible to charge O’Brien, the grand jury returned indictments against him and Campbell. The indictments were made public three months later.

State prosecutors decided not to pursue the Dow case.