US diplomats have travelled to Russia's Dagestan region to interview the parents of the Boston Marathon bombing suspects.
A delegation from the American embassy in Moscow made the journey to the North Caucasus area to interview Anzor and Zubeidat Tsarnaev.
An embassy official said the trip was in line with the co-operation between the FBI and Russian authorities over the investigation into the deadly bombings allegedly carried out by brothers Tamerlan and Dzhokhar Tsarnaev.
The parents are currently living in Dagestan, a largely Muslim region on the Caspian Sea where the family briefly lived before leaving for the US over a decade ago.
It comes amid mounting questions in the US about whether authorities there missed crucial signals that should have raised suspicions about the brothers before the bombings.
Particular interest has surrounded a six-month trip Tamerlan made in 2012 to Dagestan and Chechnya.
Russian security sources in Dagestan told the AFP news agency he was seen four times with a figure suspected of links with the Islamist underground during his visit, but there was never any reason to detain him.
Meanwhile, it has been reported Tamerlan had received welfare benefits that ended last year.
A lawyer for the suspect's wife, Katherine Russell Tsarnaev, claimed she was working up to 80 hours a week as a home health aide while Tamerlan stayed at home.
Some relatives claimed Tamerlan, 26, fell under the influence of a mystery Muslim convert and was steered towards a strict strain of Islam in the years before the attacks.
After befriending the red-bearded man known to the Tsarnaev family only as Misha, Tamerlan gave up boxing and stopped studying music, the family said.
He became vocal about his opposition to the wars in Afghanistan and Iraq, and read websites claiming that the CIA was behind the 9/11 terror attacks and Jews controlled the world.
"Somehow, he just took his brain," said Tamerlan's uncle, Ruslan Tsarni, who recalled conversations with Tamerlan's father about Misha's influence.
It was not immediately clear whether the FBI has spoken to Misha or was attempting to, but efforts by the media to identify and interview the mystery man have been unsuccessful.
The brothers' mother, Zubeidat, denied Misha had radicalised Tamerlan, telling the ABC network in the US he was "just a friend".
Throughout his religious makeover, Tamerlan maintained a strong influence over his siblings, including 19-year-old Dzhokhar, who investigators say carried out the deadly attack by his older brother's side, killing three people.
The family's claims came as new photos were released that apparently show the police stand-off that killed Tamerlan on Friday.
The images seem to have been taken from inside a house overlooking the scene of the gunfight, and show the brothers crouching near a Mercedes 4x4 they had carjacked.
One of the brothers appears to be holding a gun with both hands in front of his body.
Dzhokhar survived the shootout and was caught by the police later in the day.
He was charged on Monday with using a weapon of mass destruction to kill. He could face the death penalty if convicted.
Based on preliminary written interviews with Dzhokar in his hospital bed, US officials believe the brothers were motivated by their religious views.
Two US officials say the brothers had no tie to terrorist groups, but Tamerlan's relationship with Misha could be a clue in understanding the motives behind his religious transformation and ultimately the attack itself.
Elmirza Khozhugov, 26, the ex-husband of Tamerlan's sister, Ailina, said Tamerlan was idolised by his siblings.
"You could always hear his younger brother and sisters say 'Tamerlan said this,' and 'Tamerlan said that'. Dzhokhar loved him. He would do whatever Tamerlan would say," he said.
The Tsarnaev brothers, who were ethnic Chechens, were raised in a home that followed Sunni Islam, the religion's largest sect.
They were not regulars at the mosque and rarely discussed religion, Mr Khozhugov said.
Then, in 2008 or 2009, Tamerlan met Misha, who the family described as slightly older, heavyset and bald.
"Misha was important," Mr Khozhugov said. "Tamerlan was searching for something. He was searching for something out there."
Mr Khozhugov did not know where they had met, but believed they attended a Boston-area mosque together.
Misha was an Armenian native and a convert to Islam and quickly began influencing his new friend, family members said.
Once, Mr Khozhugov said, Misha came to the family home outside Boston and sat in the kitchen, chatting with Tamerlan for hours.
"Misha was telling him what is Islam, what is good in Islam, what is bad in Islam," said Mr Khozhugov, who said he was present for the conversation.
The conversation continued until Tamerlan's father, Anzor, came home from work around midnight.
"His father comes in and says 'Why is Misha here so late and still in our house?'. He asked it politely. Tamerlan was so much into the conversation he didn't listen."
Mr Khozhugov said Tamerlan's mother told him not to worry.
As time went on, Anzor became so concerned about his son that he called his brother, worried about Misha's influence.
"I heard about nobody else but this convert. The seed for changing his views was planted right there in Cambridge," Mr Tsarni said.
Last week, Mr Tsarni made a public appeal for Dzhokhar to turn himself in when he was still at large following his brother's death.
Since the attacks, Anzor has insisted his sons are innocent and the attacks were orchestrated to frame them. Other members of the extended family have made similar claims.
But two US officials have confirmed that Tamerlan, who was married with a young daughter, became an ardent reader of jihadist websites and extremist propaganda.
Lawyers for Katherine Tsarnaeva, Tamerlan's widow, say she is doing everything she can to assist authorities.
In a statement her legal team said Tsarnaeva, a Muslim convert, and her family were in shock when they learned of allegations against her husband and brother-in-law.
