By Daniel Lovering

BOSTON (Reuters) - Attorneys clashed in court on Tuesday over whether a friend of the accused Boston Marathon bomber was too high on marijuana to have deliberately lied to investigators about his role in the aftermath of the worst attack on U.S. soil since Sept. 11, 2001.

A federal prosecutor said Robel Phillipos, 21, lied repeatedly to FBI agents after the April 15, 2013, attack before finally confessing to entering Dzhokhar Tsarnaev's dorm room with two other men to remove a backpack containing empty fireworks shells. If convicted, Phillipos faces up to 16 years in prison for the charge of lying to investigators.

"If the defendant had just told the truth when he was first interviewed, none of us would be here today," Assistant U.S. Attorney Stephanie Siegmann said during closing arguments in U.S. District Court on Tuesday.

Phillipos' defense team said Phillipos simply could not remember what had happened when interviewed by the agents because he had smoked a lot of marijuana, and that his confession was "manufactured" on the laptop of an FBI agent.

"We're talking about anywhere between 12 and 16 hours straight of smoking (marijuana)," defense attorney Derege Demissie said.

Prosecutors say Phillipos, a student from Cambridge, Massachusetts, and two other Tsarnaev friends went to Tsarnaev's dorm room at the University of Massachusetts-Dartmouth three days after the attack, following the FBI's release of photos of the suspected bombers.

They said he lied in four separate interviews with the FBI before finally confessing in a fifth.

Phillipos' defense team said an FBI interrogator used "everything in the book" to extract the confession and made him "feel hopeless, made him feel isolated from the world."

An expert on marijuana abuse, Dr. Alan Wartenberg, testified the drug could have impaired Phillipos' cognitive abilities and memory. The attorneys also summoned former Massachusetts Governor Michael Dukakis, a friend of Phillipos' family, who said the student was "so confused he didn't know what he had said" to the FBI.

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The second Tsarnaev friend, Azamat Tazhayakov, was convicted in July of obstruction of justice, and the third, Dias Kadyrbayev, pleaded guilty to obstruction in August.

Three people were killed and more than 260 injured when two homemade bombs exploded near the finish line of the Boston Marathon. Tsarnaev, 21, awaits trial on charges that carry the death penalty. His brother Tamerlan was killed in a shootout with police days after the bombing.

(Reporting by Daniel Lovering; Editing by Richard Valdmanis, Bill Trott, Bernard Orr and Peter Cooney)