Jason Aquino, one of the three men charged in the May 2009 killing in Kirkland House, pleaded guilty yesterday to manslaughter, armed robbery, and misleading a grand jury.

Aquino, 21, was previously charged with first-degree murder in the slaying of the 21-year-old Cambridge resident Justin Cosby but negotiated an agreement with the prosecution to plead to the lesser charge of manslaughter.

According to Matthew A. Kamholtz, Aquino’s attorney, the defense and prosecution have agreed on 18 to 20 years as a recommended sentence—the high end of the possible prison time for a manslaughter conviction in Massachusetts.

According to court documents, Aquino made a plea agreement in Sept. 2009 in which he would have pleaded guilty only to accessory after the fact to an armed robbery and possession of a firearm and been sentenced to no more than five years in jail. But the state ended this agreement in March 2010 and indicted Aquino for murder, armed robbery, and intimidation of a juror.

Prosecutors said in court documents that they nullified that agreement with Aquino because he lied about his involvement in the Kirkland crime—including stating that he was in the courtyard in front of the Kirkland annex during the shooting, rather than inside the building as the testimony of three eyewitnesses established.

“Jason Aquino’s version of events was meant to drastically minimize his involvement in Cosby’s murder,” prosecutors wrote.

Under his new guilty plea yesterday, Aquino has not agreed to testify at the trial of Jabrai Jordan Copney—the defendant who is accused of firing the shot that killed Cosby.

“The case is over as far as [Aquino’s] concerned,” Kamholtz told The Crimson.

Aquino’s sentence will be determined in court on May 2—after 22-year-old Copney’s trial, which is set to begin on April 4.

Blayn Jiggetts, 21, the third defendant indicted for Cosby’s murder, has also agreed to plead guilty to the lesser count of manslaughter, the Boston Globe reported last week.

Two attorneys involved in the case confirmed that Jiggetts will testify at Copney’s trial as a condition of his plea bargain.

James D. Cormier, a criminal defense lawyer and a former Middlesex assistant district attorney, said that Jiggetts’ guilty plea may not come until after the conclusion of Copney’s trial since Jiggetts, unlike Aquino, must serve as a truthful witness at Copney’s trial as part of his plea agreement.

“They may choose to essentially hold off the plea until after he testifies to make sure he testifies to what they expect him to testify,” Cormier said. “They might delay the entire sentencing and plea process until after the testimony is given so that they can ensure he lives up to his part of the deal.”

Prosecutors say that Aquino, Copney, and Jiggetts entered the basement of Kirkland’s J-entryway on May 18, 2009, with the intention of stealing drugs from Cosby, a Cambridge resident who they claim sold marijuana to Harvard students.

According to Jiggetts’ testimony in court documents, Jiggetts asked Aquino if he could use a gun owned by Aquino for the “drug rip” that he and Copney were planning at Harvard. Aquino said he needed at least $500 to pay his rent and asked to join in on the robbery, the documents say.

Aquino allegedly showed Jiggetts two guns but said that he could not use one because it had “four or five bodies on it.”

Brittany J. Smith, the fourth defendant charged in connection to the shooting, was a senior in Lowell House and Copney’s girlfriend at the time of the shooting. Prosecutors allege that she hid the murder weapon after the crime was committed and helped the three men flee to New York City, among other crimes.

—Staff writer Xi Yu can be reached at xyu@college.harvard.edu.

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