(Reuters) - Former New England Patriots football player Aaron Hernandez witnessed the killing of Odin Lloyd in 2013, but did not commit the murder himself, one of his attorneys claimed in the closing statements of his trial on Tuesday.

"He was a 23-year-old kid who witnessed ... a shocking killing committed by somebody he knew," said defense attorney James Sultan. "He really didn't know what to do. So he just put one foot in front of the other."

Hernandez, 25, has pleaded not guilty to murder and firearms charges in the death of Lloyd, a semiprofessional football player who was dating his fiancée's sister. If convicted, he faces a maximum penalty of life in prison.

Prosecutors say the former tight end and two friends, Ernest Wallace and Carlos Ortiz, picked up Lloyd at his Boston home before dawn on June 17, 2013, and drove him to an industrial park near Hernandez's house in North Attleboro, Massachusetts, where they shot him to death.

But his defense team has said the prosecution's case failed to prove that Hernandez pulled the trigger. Sultan added on Monday that Hernandez and Lloyd were friends who shared "a passion for marijuana."

"Obviously they were friends," Sultan told the jury. "They were future brothers-in-law."

Wallace and Ortiz have also pleaded not guilty to murder charges and will be tried separately. Both men used the drug PCP, according to attorneys and court documents, and the defense team has sought to link use of the drug with violent, unpredictable behavior in an apparent attempt to shift blame for the murder on the pair.

Investigators say Lloyd was shot multiple times with a Glock .45-caliber handgun that has not been found.

Sultan said that after nine weeks of testimony, prosecutors "still haven't proved to you who killed Odin Lloyd." During the trial, which started in late January, the prosecution called 131 witnesses while the defense called three.

Sultan also attacked the investigation, saying a shell casing removed from a rental car returned by Hernandez had been contaminated with his client's DNA from a piece of chewing gum that was stuck to it.

Hernandez had a $41 million contract with the Patriots. He was cut after being charged with Lloyd's killing in June 2013. Prosecutors are expected to deliver their closing arguments later on Tuesday.