As the investigation into why and how the Boston bombers unleashed their deadly attacks continues, the focus has centered to an elusive and mysterious Muslim convert known only as Misha.
Family members of Tamerlan Tsarnaev describe Misha as the guiding influence in the elder bomber developing radicalized views - but to date it is believed that law enforcement and the media have failed to find him.
Speculation as to who Misha is wildly varies, with some suggesting he is the mastermind behind the marathon bombings while others think that he could be a Russian spy - sent to identify and keep tabs on young men like Tamerlan who are at risk of turning to militant Islam.
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Bombing Attack: Tamerlan Tsarnaev, 26, and his brother, Dzhokhar A. Tsarnaev, 19, approximately 10-20 minutes before the blasts that struck the Boston Marathon
All that is known about him is that he is an Armenian man in his 30s with distinctive red beard and that he has disappeared - no longer living in the Cambridge, Massachusetts area.
However, family members have been telling reporters that in the years before the Boston marathon bombings, Tamerlan, 26, fell under the strong influence of a new friend, a Christian who converted to Islam and who steered the religiously apathetic young man towards adopting strong Islamic views.
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'It started in 2009. And it started right there, in Cambridge,' said Tamerlan's uncle Ruslan Tsarni to CNN from his home in Maryland.
'This person just took his brain. He just brainwashed him completely.'
Place of worship: A banner reading 'United We Stand For Peace on Earth' stands outside the Islamic Society of Boston mosque in Cambridge, Massachusetts, where Tamerlan Tsarnaev attended
Goofy: Tamerlan Tsarnaev practices boxing at the Wai Kru Mixed Martial Arts center in April 2009 in Boston, Massachusetts
The moment of the explosion at the Boston Marathon finish line: Three people were killed and a further 264 injured in the two blasts, which happened last Monday afternoon 
Innocence: The Tsarnaev siblings as children. Dzhokhar, center, stands in front of his older brother Tamerlan as they are accompanied by their sisters
Kerry: Boston suspect 'returned from Chechnya with intent to...
Tamerlan Tsarnaev was killed in a police shootout Friday. Dzhokhar Tsarnaev was charged Monday with using a weapon of mass destruction to kill, and he could face the death penalty if convicted.
Under the tutelage of a friend known to the Tsarnaev family only as Misha, Tamerlan gave up boxing and stopped studying music, his family said. He began opposing the wars in Afghanistan and Iraq.
He turned to websites and literature claiming that the CIA was behind the terrorist attacks of September 11th, 2001, and Jews controlled the world.
According to Ruslan, Tamerlan's radicalization happened right under the nose of his mother, Zubeidat Tsarnaeva.
Speaking to MailOnline last week, Ruslan said that Misha was around 30-years-old and that he was an Armenian who, unusually for such a largely Christian people, had converted to Islam.
Tamerlan's relationship with Misha could be a clue in understanding the motives behind his religious transformation and, ultimately, the attack itself.
Indeed, the struggle to understand why the Tamerlan and his brother Dzhokhar carried out their attacks deepened when U.S. officials said that Tamerlan had no tie to terrorist groups.
As authorities try to piece together that information, they are touching on a question asked after so many terrorist plots: What turns someone into a terrorist?
Troubled family: This black and white photo shows Tamerlan as a baby, with his father Anzor (left), mother Zubeidat and uncle Muhamad Suleimanov
The brothers emigrated in 2002 or 2003 from Dagestan, a Russian republic that has become an epicenter of the Islamic insurgency that spilled over from the region of Chechnya.
They 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, said Elmirza Khozhugov, 26, the ex-husband of Tamerlan's sister, Ailina.
Then, in 2008 or 2009, Tamerlan met Misha, a slightly older, heavyset bald man with a long reddish beard. Khozhugov didn't know where they'd 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, 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 Khozhugov, who said he was present for the conversation. 'This is the best religion and that's it. Mohammed said this and Mohammed said that.'
According to Uncle Ruslan, Tamerlan, who was an accomplished boxer, began his radicalization by 2007 - right under his mother's nose
Boston Marathon bombing suspects Dzhokhar (back) and Tamerlan Tsarnaev were revealed in this handout photo during an FBI news conference in Boston on April 18
The conversation continued until Tamerlan's father, Anzor, came home from work.
'It was late, like midnight,' Khozhugov said. '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.'
Khozhugov said Tamerlan's mother, Zubeidat, told him not to worry.
'Don't interrupt them,' Khozhugov recalled the mother saying. 'They're talking about religion and good things. Misha is teaching him to be good and nice.'
As time went on, Tamerlan and his father argued about the young man's new beliefs.
'When Misha would start talking, Tamerlan would stop talking and listen. It upset his father because Tamerlan wouldn't listen to him as much,' Khozhugov said.
'He would listen to this guy from the mosque who was preaching to him.'
Anzor became so concerned that he called his brother, worried about Misha's effects.
'I heard about nobody else but this convert,' Tsarni said. 'The seed for changing his views was planted right there in Cambridge.'
Zubeidat Tsarnaeva, mother of Tamerlan and Dzhokhar Tsarnaev, the two men who set off bombs near the Boston Marathon finish line on April 15, 2013 in Boston, walks with an unidentified man near her home in Makhachkala, Dagestan (left) while (right) Anzor Tsarnaev, who identifies himself as the father of Dzhokhar and Tamerlan Tsarnaev, the two suspects in the Boston Marathon bombings, gives an interview in Makhachkala
It was not immediately clear whether the FBI has spoken to Misha or was attempting to.
While Misha is a very common name across the former Soviet Union, Dan Amira makes the point that 'there can't be that many bald, red-bearded Armenian Muslims in Boston.
Respected national security writer Laura Rozen took to Twitter to speculate that Misha could be 'the kind of mole Russia plants to keep on eye on emigre communities of concern.'
Indeed, she theorizes that Misha could even be the souce that tipped off Russian security sevices to Tamerlan's conversion to radical Islam in 2011.
Tsarnaev became an ardent reader of jihadist websites and extremist propaganda, two U.S. officials said. He read Inspire magazine, an English-language online publication produced by al-Qaida's Yemen affiliate.
Tamerlan loved music and, a few years ago, he sent Khozhugov a song he'd composed in English and Russian. He said he was about to start music school.
Six weeks later, the two men spoke on the phone. Khozhugov asked how school was going.
'I quit,' Tamerlan said.
'Why did you quit?' Khozhugov asked. 'You just started.'
'Music is not really supported in Islam,' he replied.
'Who told you that?'
'Misha said it's not really good to create music. It's not really good to listen to music,' Tamerlan said, according to Khozhugov.
Suspects: Tamerlan Tsarnaev, left, was killed in a police shoot-out on Friday. His younger brother Dzokhar, right, was charged on Monday with using a weapon of mass destruction to kill and could face death if convicted
Boss: Zubeydat Tsarnaev, mother of the terrorist suspect brothers Dzokhar and Tamerlan, is accused of letting a radical cleric preach to her boys in their family kitchen
Tamerlan took an interest in Infowars, a conspiracy theory website. Khozhugov said Tamerlan was interested in finding a copy of the book 'The Protocols of the Elders of Zion,' the classic anti-Semitic hoax, first published in Russia in 1903, that claims a Jewish plot to take over the world.
'He never said he hated America or he hated the Jews,' Khozhugov said. 'But he was fairly aggressive toward the policies of the U.S. toward countries with Muslim populations. He disliked the wars.'
One of the brothers' neighbors, Albrecht Ammon, recently recalled an encounter in which Tamerlan argued about U.S. foreign policy, the wars in Afghanistan and Iraq, and religion.
Ammon said Tamerlan described the Bible as a 'cheap copy' of the Quran, used to justify wars with other countries.
'He had nothing against the American people,' Ammon said. 'He had something against the American government.'
Khozhugov said Tamerlan did not know much about Islam beyond what he found online or what he heard from Misha.
'Misha was important,' he said. 'Tamerlan was searching for something. He was searching for something out there.'
But, the mother of the alleged Boston bombers denied reports that her sons had been radicalized by a mysterious convert to Islam named Misha.
'Nonsense. He was just a friend,' Zubeidat Tsarnaeva told ABC News by phone today shortly before she sat down with FBI investigators for a second day of interviews here in the restive region of Dagestan, in southern Russia.
Tsarnaeva said Misha knew a lot about Islam and that it was interesting to learn from him, but denied his views were extreme.
Twin detonations went off near the finish line of the 117th Boston Marathon on April 15, 2013
She said their relationship with Misha, an Armenian with a red beard whose identity and full name remain a mystery, was short because he moved to another part of the United States since. She would not say where.
Throughout his religious makeover, Tamerlan maintained a strong influence over his siblings, including Dzhokhar, who investigators say carried out the deadly attack by his older brother's side, killing three and injuring 264 people.
'They all loved Tamerlan. He was the eldest one and he, in many ways, was the role model for his sisters and his brother,' said Elmirza Khozhugov, 26, the ex-husband of Tamerlan's sister, Ailina.
'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.
'Even my ex-wife loved him so much and respected him so much,' Khozhugov said. 'I'd have arguments with her and if Tamerlan took my side, she would agree: 'OK, if Tamerlan said it.''
Khozhugov said he was close to Tamerlan when he was married and they kept in touch for a while but drifted apart in the past two years or so.
There was smoke and panic in the street as emergency personnel responded to the scene after two explosions went off near the finish line of the 117th Boston Marathon on April 15, 2013
He spoke to the AP from his home in Almaty, Kazakhstan. A family member in the United States provided the contact information.
'Of course I was shocked and surprised that he was Suspect No. 1,' Khozhugov said, recalling the days after the bombing when the FBI identified Tamerlan as the primary suspect.
'But after a few hours of thinking about it, I thought it could be possible that he did it.'
Based on preliminary written interviews with Dzhokar in his hospital bed, U.S. officials believe the brothers were motivated by their religious views. It has not been clear, however, what those views were.
Katherine Russell, the widow of Boston Marathon bombing suspect Tamerlan Tsarnaev, leaves her lawyer's office
History: Bombing suspect Tamerlan Tsarnaev (left) displayed a disturbing pattern of verbal abuse and violence toward women, including his wife, Katherine Russell (right)
Tamerlan Tsarnaev practices boxing at the Wai Kru Mixed Martial Arts center in April 2009 in Boston, Massachusetts
This comes as FBI agents question members of the mosque where one of the suspected Boston bombers attended services.
Tamerlan Tsarnaev, who was killed in a shoot-out with police on Friday, was still going to prayer services at a Cambridge, Massachusetts, mosque just weeks before the bombing.
Mosque spokesman Yusuf Vali said the 26-year-old had previously caused disturbances there, and imams had threatened to ban him following a confrontation during a prayer service.
Mr Vali told NBC News that as soon as they learned of Tamerlan's alleged involvement 'we immediately called law enforcement and said: "Listen, we've got folks who knew him and if you need any information, we're here."'
'Those folks have already met with the FBI,' he added.