Since then, prosecutors have been ramping up efforts to identify actions taken by lawmakers that could be criminal, such as accepting probation jobs for family and friends or even campaign donations from probation employees in exchange for voting to increase the agency’s funding or to vote for a certain candidate for speaker. It is a violation for lawmakers to accept something of value in exchange for any official act.

DeLeo, who has described O’Brien as a friend, was one of the most successful legislators at helping people obtain jobs at probation, including his godson, Brian Mirasolo, who, at age 28, became the youngest chief probation officer in Massachusetts history.

But DeLeo strongly denies that he used his connections with O’Brien to win votes from fellow Democratic legislators for the speakership, which brought him a pay increase of more than $30,000 along with a larger staff and far greater influence over state policy.

“There is no way any individual can honestly testify that jobs were traded for votes for speaker, because it did not happen,” said DeLeo spokesman Seth Gitell.

Paul F. Ware, the independent counsel who issued a damning report on the hiring practices at the Probation Department in 2010, said he surmises that federal prosecutors are now trying to negotiate a deal with O’Brien to get his cooperation in the investigation of politicians’ culpability.

“In terms of a criminal investigation, I think it’s predictable that the US attorney focus on political figures who may have had a lot more to gain” from Probation Department patronage than O’Brien, said Ware, a former federal prosecutor who is now a partner at Goodwin Procter in Boston.