By Scott Malone

BOSTON (Reuters) - A friend of the Boston Marathon bombing suspect could not have lied to the FBI about visiting his dorm room and removing a backpack because he was too intoxicated by marijuana to remember what he did that day, his attorney said on Monday.

Robel Phillipos, a 21-year-old resident of Cambridge, Massachusetts, is one of three people who prosecutors say went to accused bomber Dzhokhar Tsarnaev's room at the University of Massachusetts at Dartmouth on April 18, 2013, after the FBI released images of the Tsarnaev brothers, and removed a backpack containing empty fireworks shells.

U.S. prosecutors said in their opening statements that Phillipos had repeatedly lied to FBI interviewers about his actions that day. His attorney, Derege Demissie, contended that his client had no memory of his actions that day.

"There will be ample evidence ... much of it is undisputed that Robel spent the entire day of April 18 smoking marijuana, highly intoxicated, with several friends," Demissie said. "His memory is jumbled, confused and completely discombobulated."

Assistant U.S. Attorney John Capin painted a contrasting picture, arguing that Phillipos lied after the FBI released photos of Tsarnaev and his older brother, Tamerlan Tsarnaev, in a request for the public's help in identifying them.

"What did these three men do when they recognized their friend as a suspect in the Boston Marathon bombing investigation? They went to his dorm room," Capin said.

Defense attorneys may have difficulty convincing jurors of Phillipos' contention he was too stoned to know what he did that day, given the high passions around the April 15, 2013, bombing that killed three people and injured more than 260, said Robert Bloom, a professor at Boston College Law School.

"Not remembering what you did is not a lie," Bloom said. "Whether or not the jury is going to buy it is another matter."

The other two friends were Kazakh exchange students Azamat Tazhayakov and Dias Kadyrbayev. Tazhayakov was found guilty of obstruction of justice for taking the backpack and tossing it into a dumpster, and Kadyrbayev pleaded guilty to the same charges.

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Tazhayakov, who did not testify during his own July trial, is due to take the stand as a prosecution witness later in the trial, prosecutors said.

Phillipos, a U.S. citizen who has been largely confined to his home since his arrest last year, could face up to 16 years in prison if convicted of lying to investigators. He did not speak during Monday's proceedings at U.S. District Court in Boston.

Prosecutors contend that he initially denied entering Tsarnaev's dorm room hours after the FBI released photos of the two brothers, who authorities say set off the two homemade bombs near the Boston Marathon finish line.

Around the time of the visit to Tsarnaev's dorm room, the brothers were preparing to flee Boston, and they fatally shot a university police officer in an attempt to steal his gun, prosecutors say. Later that night, Tamerlan Tsarnaev died after a shootout with police.

Phillipos, Tazhayakov and Kadyrbayev are not charged with playing any role in the bombing.

The surviving Tsarnaev, 21, faces the death penalty if convicted of carrying out the bombing.

(Reporting by Scott Malone; Editing by Mohammad Zargham and Peter Cooney)