(Adds fresh quotes; prosecution and defense rest)
By Daniel Lovering
BOSTON, July 14 (Reuters) - A friend of the accused Boston Marathon bomber, charged with obstructing the probe into the blasts, and his roommate "simultaneously" told the FBI a backpack belonging to the suspect had been thrown into a dumpster, an agent said Monday.
Azamat Tazhayakov is the first of three friends of Dzhokhar Tsarnaev to face trial on charges of interfering with the investigation into the blasts by removing a laptop and the backpack, which contained empty fireworks casings, from Tsarnaev's room three days after the April 15, 2013, attacks that killed three people and injured 264.
FBI Special Agent John Walker interviewed Tazhayakov, his roommate and fellow Kazakh exchange student Dias Kadyrbayev and a woman described as Kadyrbayev's girlfriend in the days after the attacks, at one point recovering a baseball cap and ash tray taken from Tsarnaev's room at the University of Massachusetts at Dartmouth.
On April 20, the second day of interviewing the friends, Walker told the pair that the FBI was trying to find the backpack.
"They told me simultaneously that the backpack had been placed in the dumpster and that a refuse truck had removed the dumpster from the location the previous day," he said.
Walker acknowledged that Tazhayakov had not said that he had thrown the backpack into the dumpster, and that it was Kadyrbayev who described the dumpster and its location on the grounds of the apartment complex in New Bedford, Massachusetts, about 50 miles (80 km) south of Boston.
Other FBI agents recovered the backpack at a landfill several days later. It contained fireworks with the gunpowder removed, a spiral notebook, a jar of Vaseline - which an FBI forensic examiner has testified can be used to make an explosive device - and a homework assignment from an ethics class in which Tsarnaev was enrolled.
Prosecutors rested their case following Walker's testimony on Monday, the defense presented no witnesses and U.S. District Judge Douglas Woodlock set closing arguments for Wednesday.
FBI agent Farbod Azad testified last week that Tazhayakov told him he and Kadyrbayev and a third man, Robel Phillipos of Cambridge, Massachusetts, had removed the backpack and a laptop from Tsarnaev's dorm room.
Tazhayakov's attorneys say their client never touched the backpack or the fireworks, and that it was Kadyrbayev who threw them out. Kadyrbayev's girlfriend, Bayan Kumiskali, said in a videotaped deposition shown in court that she became upset when she learned of the backpack and asked her boyfriend to "get it out of the apartment."
The attorneys argued before the trial that Tazhayakov's statements during an initial FBI interview, which began April 19 and ran into the next morning, should not be admitted at trial because he had not believed he was free to go at the time.
Tazhayakov could face up to 25 years in prison if convicted of obstruction of justice and conspiracy. Kadyrbayev faces the same charges. Phillipos is accused of the lesser charge of lying to investigators.
Tsarnaev is awaiting trial on terrorism charges. (Editing by Scott Malone, Doina Chiacu and Jim Loney)