Jason F. Aquino, one of three men implicated in the fatal 2009 shooting of Cambridge resident Justin Cosby in the basement of Kirkland House, was sentenced Monday to 18 to 20 years in prison.

Aquino pleaded guilty in March to manslaughter, which carries a maximum penalty of 20 years in prison. He was originally charged with first-degree murder but was permitted to plea to a lesser charge in exchange for cooperating with the prosecution.

Aquino made a plea agreement in September 2009 which would have guaranteed him no more than five years in prison. But prosecutors revoked the agreement when they found that he had made false statements to the grand jury.

Most notably, Aquino originally said that he had waited in the courtyard in front of the Kirkland annex while the other two murder defendants, Jabrai Jordan Copney and Blayn Jiggetts, led Cosby into Kirkland’s J-entryway. There, Cosby was held up for marijuana and then shot when he refused to hand over the drugs.

Several eyewitnesses later said that Aquino was not standing in the courtyard during the incident; in fact, they said, three men followed the bleeding victim out of the entryway.

As a consequence of those lies, Superior Court Judge Howard Whitehead sentenced Aquino on Monday to four to five years in prison for willfully misleading a grand jury—to be served concurrently with the 18 to 20 years for manslaughter, according to Middlesex District Attorney’s office spokesperson Cara O’Brien and to Aquino’s attorney Matthew A. Kamholtz.

Though prosecutors have named Copney as the man who fired the three shots—one of which fatally struck Cosby in the abdomen—all three men were originally charged with first-degree murder, based on their plan to rob Cosby of marijuana at gunpoint. The felonymurder theory, one of three bases for first-degree murder, legally permits someone to be found guilty of the most serious murder if a killing occurs during the commission or intended commission of any felony punishable by up to life in prison.

Jiggetts has also pleaded guilty to manslaughter and has been promised a sentence of 9 to 12 years, based in large part on his cooperation as a witness against Copney at the accused gunman’s recent trial.

O’Brien said that Jiggetts’ sentencing will take place after the trial of the fourth defendant in the case, Brittany J. Smith, who was a Harvard senior when the murder occurred.

Aquino was not called as a witness in Copney’s trial.

After a three-week trial, Copney was found guilty of first-degree murder last month. He received a sentence of life without parole, the mandatory penalty for first-degree murder.

Discussing Aquino, Jiggetts, and Copney’s widely varying sentences, Kamholtz said, “I think the sentences in part represent the level of culpability of the defendant but not entirely.... I think that an effort was made on the part of the DA’s office to get the information that they needed and to assess the relative culpability of the defendants.”

He called his client’s sentence “fair.”

Smith, the remaining defendant, was a Lowell House resident and Copney’s girlfriend at the time of the shooting. She has been charged with accessory after the fact of murder and five other counts based on allegations that she hid the murder weapon and helped the killers flee Cambridge.

In a statement following Aquino’s sentencing, Middlesex District Attorney Gerard T. Leone Jr. ’85 said, “Our efforts continue as we turn our attention to the sentencing of Blayn Jiggetts, and the trial of Brittany Smith.”

Smith has filed a motion asking that the charges against her be dismissed, based on an earlier cooperation agreement which prosecutors say she has violated by lying to investigators.

Middlesex First Assistant Clerk Matthew Day said that Judge Diane M. Kottmyer will rule on the motion before or at Smith’s scheduled court appearance on Monday.

—Xi Yu contributed reporting to this story.

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