Federal prosecutors are scaling back their case against former state Probation Department boss John O’Brien, dropping two of 10 mail fraud charges that form the bulk of the case against O’Brien and two former deputies.

Fred Wyshak, the lead prosecutor in O’Brien’s federal corruption trial, said Monday that the US Attorney’s office won’t pursue mail fraud charges related to the hiring of Lisa Martin and the promotion of Antonio Mataragas. After an abbreviated court session Monday, Wyshak said prosecutors were trying to streamline their case against O’Brien and two former top aides, Elizabeth Tavares and William Burke III. “At some point, you have to try to be efficient and get the case in without unfairly elongating the trial,” Wyshak said. The O’Brien trial began in early May, and is expected to stretch well into the summer.

The government is alleging that O’Brien, Tavares, and Burke ran the state Probation Department like a criminal organization. They claim the defendants created a rigged hiring system inside Probation that existed to curry favor with powerful state lawmakers, handing out “jobs like lollipops” to politically connected job-seekers, turning Probation jobs into “political currency,” and papering over the political hiring system with falsified hiring documents.

Prosecutors have assembled a sprawling set of 39 allegedly crooked Probation hires and promotions that, they claim, advanced a complex conspiracy and racketeering scheme. Most of these hires and promotions are being charged to prove the alleged racketeering. Ten of them, however, are also being prosecuted as alleged mail fraud acts. Prosecutors claim that when Probation officials mailed rejection letters to job-seekers who lost out to politically connected candidates, they committed mail fraud. Wyshak said Monday that his team is giving up on two of these 10 mail fraud charges: Martin’s 2008 hiring as a Probation officer, and Mataragas’s 2008 promotion.

According to the indictment of O’Brien, Tavares, and Burke, Martin landed a job in the Worcester County Probate and Family court in March 2008, after being sponsored by a state rep and a member of the state judiciary. One of O’Brien’s attorneys, Stellio Sinnis, identified Martin’s sponsor as Rep. James O’Day earlier in the trial. The Globe previously identified Martin’s judicial sponsor as Edward J. Reynolds, a retired state judge. Sinnis has argued that there was nothing improper about Martin’s hiring because she was qualified for the job.

Mataragas appears twice in the O’Brien indictment – once upon his 2007 hiring to the Middlesex County Juvenile Court, and again when he was promoted to a job in the Peabody District Court, in 2008. Jurors have already heard testimony from Francine Gannon, the constituent services director for Senate President Therese Murray, who described an effort by Murray and former Sen. Fred Berry to secure a promotion for Mataragas. Prosecutors won’t pursue mail fraud charges related to Mataragas’s promotion, but they’re still claiming that his original hiring was improper.

The judge in the O’Brien case, William Young, has repeatedly said he believes mail fraud to be the meat of the case against O’Brien. So in the sense that prosecutors are walking away from one-fifth of the central element of their case, Monday’s announcement was significant. But it’s not clear that Wyshak, the lead prosecutor, sees things that way. Wyshak said Monday that he’d been told by Judge Young not to start arguing the government’s wider racketeering case against O’Brien, Tavares, and Burke until he’d finished building his central mail fraud case. In exchanges with Judge Young, Wyshak has appeared eager to press his broader racketeering case. With two fewer mail fraud charges on the table, Wyshak expects to get to work trying to prove the wider racketeering charges next week.