Federal prosecutors investigating the scandal-plagued Probation Department are focusing on Democratic legislators who may have benefited from the agency’s allegedly rigged hiring system, including House Speaker Robert A. DeLeo, one of the most influential politicians in helping people get jobs under former commissioner John J. O’Brien.

Prosecutors have subpoenaed several lawmakers to testify before a secret grand jury considering criminal charges in recent weeks, including Representative John Rogers, the Norwood lawmaker DeLeo defeated to become House speaker in 2009. Rogers’s supporters have long suspected that O’Brien helped DeLeo win by giving jobs to people close to legislators backing DeLeo, a charge both men deny.

People with direct knowledge of the grand jury proceedings in Worcester say that prosecutors have also called at least two legislators who voted for DeLeo for speaker after people close to them received probation jobs.

“Yes, I was called to testify and I cooperated fully,” said Representative James J. O’Day of West Boylston, one of the two. He has said that he planned to vote for DeLeo even before probation hired one of his colleagues in 2008.

US Attorney Carmen M. Ortiz has been investigating the Legislature’s role in politicizing the hiring process at the Probation Department for months, even calling former House speaker Salvatore F. DiMasi from his prison cell in Kentucky to testify before the grand jury last winter. But in March, when Ortiz announced the first criminal indictments from the 20-month-long inquiry, only O’Brien and two former top lieutenants were named.

“This is just one step in the ongoing investigation,” Ortiz said at the time.