While one defendant implicated in the 2009 murder in Kirkland House pleaded guilty Thursday and a second is set to go to trial next week, the status of Blayn Jiggetts, the third man accused of the crime, remains unclear because his case file has been impounded by a Superior Court judge.

Jiggetts has been charged with first-degree murder, armed robbery, and three other counts.

When a court orders material impounded, it is no longer publicly available, and Middlesex Superior Court has impounded the entirety of Jiggetts’ case file.

John A. Amabile—the attorney for Jabrai Jordan Copney, the man charged with firing the shot that killed 21-year-old Cambridge resident Justin Cosby—said he plans to file a motion today to have Jiggetts’ case file unimpounded. “I want to get my hands on the docket,” he said. “The DA, I don’t believe, will oppose that.”

According to lawyers familiar with the case and The Boston Globe, Jiggetts has agreed to plea guilty to manslaughter, which carries a maximum sentence of 20 years, rather than first-degree murder, which would come with a mandatory life sentence. In return, Jiggetts has agreed to testify during Copney’s upcoming murder trial.

Jason Aquino, another man indicted on charges related to the murder, accepted a similar agreement and pleaded guilty to manslaughter last week. His attorney said that he has agreed to a recommended sentence of 18 to 20 years in prison. Aquino has not agreed to testify against Copney.

The schedule available at the Middlesex Superior Court clerk’s office states that Jiggetts will stand trial beginning on April 25, and the docket provided by the clerk’s office—which lists no activity in the case since Aug. 2010—denotes Jiggetts’ plea to all counts as not guilty. However, since Jiggetts’ case file has been impounded by the court, reporters are prevented from viewing it, and Amabile said that Jiggetts has already pleaded guilty in secret.

Amabile said that Daniel J. Bennett ’85, a prosecutor in the case, told him that Jiggetts has pleaded guilty.

Both prosecutors in the case, Middlesex District Attorney’s Office Spokesperson Cara O’Brien, and Jiggetts’ attorney George E. Murphy Jr. ’70 all declined to state whether Jiggetts has pleaded guilty. Several cited the fact that his case file is impounded as their basis for not discussing the current status of the case.

“They sort of snuck him in there to plead him out,” Amabile said. “For some reason they wanted to keep this under wraps. I don’t know why.”

Karen A. Colucci, a criminal defense lawyer and former Middlesex assistant district attorney, said that it would be “pretty unusual” for a guilty plea to be kept secret.

“I have to say I am surprised—usually if somebody pleads guilty, it’s in open court,” Colucci said. “Usually a plea is a matter of public record—documents in the file can be impounded, but the fact that somebody pled is a matter of public record.”

Several people with knowledge of Superior Court procedure said that a case file is typically impounded when it contains information that might endanger the safety of a witness or victim. This is most common in sexual assault cases, they said.

Prosecutors allege that Jiggetts, 21, and the two other men arranged to meet Justin Cosby—a 21-year-old Cambridge resident who court documents say sold drugs to Harvard students—at Harvard on May 18, 2009, with the intention of stealing drugs from Cosby. In the ensuing meeting in the basement of Kirkland’s J-entryway, Copney allegedly shot Cosby, using a gun that Jiggetts obtained prior to the men’s trip to Cambridge and loaded before the crime.

—Kerry M. Flynn, Hana N. Rouse, and Xi Yu contributed reporting to this article.

—Staff writer Julie M. Zauzmer can be reached at jzauzmer@college.harvard.edu.