Three men have been taken into custody as part of the investigation into last month's Boston Marathon bombings.

Two of the men were university classmates of 19-year-old Dzhokhar Tsarnaev.

Kazakhstani nationals Azamat Tazhayakov, 19, and Dias Kadyrbayev, 19, were charged with conspiring to obstruct justice

A US citizen named Robel Phillipos was charged with making false statements to investigators.

Dzhokhar Tsarnaev was last week charged with planting home-made pressure cooker bombs at the marathon finish line on 15 April along with his older brother, Tamerlan Tsarnaev.

Papers filed in federal court said authorities charged Mr Tazhayakov and Mr Kadyrbayev with conspiracy to obstruct justice by throwing away a backpack containing fireworks and a laptop computer belonging to Dzhokhar Tsarnaev.

US prosecutors also charged Mr Phillipos, with making false statements to investigators after the bombings, which killed three people and injured 264 others.

The three men were handed into the custody of the US Marshals Service after their court appearance.

Dzhokhar Tsarnaev, who attended the University of Massachusetts at Dartmouth, is being held at a prison hospital where he is recovering from gunshot wound sustained in a gun battle with police. His older brother died in the gunfight.

Last week police officials were seen searching dumps in southeast Massachusetts.

Mr Kadyrbayev's lawyer said his client was being held for violations of his student visa.

Robert Stahl, said his client was "not a target" of the bombing investigation, but declined to comment on any other specifics.

He said his client had cooperated fully with investigators and "wants to go home to Kazakhstan".

Elsewhere, the widow of Tamerlan Tsarnaev wants a medical examiner to release his remains to the Tsarnaev family, her lawyer has said.

"It is Katherine Russell's wish that his remains be released to the Tsarnaev family, and we will communicate her wishes to the proper authorities," Amato DeLuca said in a statement.

Investigators have questioned Ms Russell as they seek clues about how the brothers allegedly built the bombs and whether they had help.

The 24-year-old widow, who returned to her family in Rhode Island days after the blasts, has spent hours meeting with FBI agents at her home and at her lawyer's office in Providence.

"In the coming days, Katherine will continue to meet with law enforcement, as she has done for many hours over the past week, and provide as much assistance to the investigation as she can," Mr DeLuca said.

On Monday, FBI investigators took DNA samples from the Russell home in North Kingstown.

Officials are trying to determine who else may have handled the pressure cookers that contained the bombs after they found a woman's DNA on at least one of them, an official said.

Also on Monday, a post-mortem examination on Tamerlan Tsarnaev determined precisely how he died after a bloody shootout with police.

However, the results cannot be made public until the body is claimed, a spokesman for the Massachusetts Medical Examiner said.

Authorities and the public have been waiting to learn whether he was killed by police or whether he was run over by his younger brother when Dzhokhar fled in a vehicle the pair had allegedly stolen.