By Scott Malone

BOSTON (Reuters) - Two college friends of the Boston Marathon bomber were sentenced to prison on Friday for removing a backpack containing empty fireworks shells from Dzhokhar Tsarnaev's dorm room amid a massive manhunt three days after the deadly 2013 attack.

Kazakh exchange student Azamat Tazhayakov was sentenced to 3-1/2 years for obstructing justice by taking, with his roommate, Tsarnaev's pack and dropping it in a dumpster by the off-campus apartment they shared.

Robel Phillipos, of Cambridge, Massachusetts, was sentenced to three years in prison for lying to investigators about the incident, which occurred in the hours after the FBI released images of Tsarnaev and his older brother, Tamerlan, identifying them as suspects.

A third friend, Dias Kadyrbayev, also of Kazakhstan, was sentenced on Tuesday to six years after pleading guilty to obstructing the investigation into the attack that killed three people and injured 264.

The United States never claimed that the three men had any advanced knowledge of the bombing plan, one of the highest-profile attacks on U.S. soil since Sept. 11, 2001..

Tazhayakov and Kadyrbayev made statements of apology to the court for their crimes. Phillipos opted not to speak, citing a planned appeal of his conviction.

"It just makes me sick what Dzhokhar did," Tazhayakov told the court, holding back tears. "I didn't go there to the dorm room because I made connection that Dzhokhar was some jihadist. I never thought about it. At that moment I saw that one of my friends was alleged bomber and I didn't know if it was true."

Prosecutors had sought a four-year sentence for Tazhayakov, which they said reflected his willingness to testify against Tsarnaev. They were seeking a five-year, three-month sentence for Phillipos.

Phillipos' mother, Genet Bekele, suggested in a written statement to reporters that her son was a victim of overzealous prosecution.

"We are very disappointed with the judge's decision," Bekele said. "My son was caught in a political storm in a situation that was incredibly hard to deal with."

All three men, along with Tsarnaev, were 19-year-old students at the University of Massachusetts at the time of the bombing.

Tsarnaev was sentenced to death last month by the same jury that found him guilty of the April 15, 2013, attack.

During their trials last year, lawyers for Tazhayakov and Phillipos painted their clients as naive, marijuana-smoking teenagers who did not understand the consequences of removing the backpack from Tsarnaev's dorm room.

Kadyrbayev later threw the backpack into a dumpster by the apartment he shared with Tazhayakov. FBI investigators recovered it from a landfill several days later.

Empty fireworks shells found by the backpack were shown as evidence at Tsarnaev's trial.

U.S. District Judge Douglas Woodlock told Phillipos he thought little of his defense strategy, that he had been too high on marijuana at the time of the dorm visit to remember his actions and thus unable to lie about the.

Noting that the United States is in the midst of a debate over whether to decriminalize or legalize the drug, he said Phillipos' case illustrated some limits.

"What's being and has been presented and rejected by the jury is the idea that marijuana use is a defense," Woodlock said. "It seems to that has to be addressed head on by saying, if we give the right to legal use of marijuana, we're not giving a defense in a criminal case."

Unlike Tazhayakov and Kadyrbayev, who have been in federal custody for more than two years, Phillipos has been on house arrest since he was first charged. He was ordered to surrender to the U.S. Marshals Service on July 24.

(Reporting by Scott Malone; Editing by Leslie Adler and Tom Brown)