"MAMA, why are you worrying?"

Tamerlan Tsarnaev reassured his mum over the phone that he was OK and unharmed in the Boston bombing. He was laughing when he said it, the Wall Street Journal reports.

A few days later he spoke to her again, but that conversation took on a markedly different tone.

"The police, they have started shooting at us, they are chasing us," he said. "Mama, I love you."

The phone dropped out and hours later Zubeidat Tsarnaeva's daughter called her in tears to tell her that Tamerlan, the 26-year-old suspected ringleader of the Marathon bombings, was dead, according to the Wall Street Journal's report.

As authorities wait to question his younger brother and alleged co-conspirator, Dzhokhar, 19, more emerges about the character of two brothers driven to a heinous act of terrorism.

The week of terror in Boston started when the brothers allegedly carried two backpacks containing pressure-cooker explosives into the crowd near the finish line of the Boston Marathon. Moments later the backpacks exploded, killing three people and injuring more than 170.

On Friday, more mayhem ensued when an MIT police officer was shot dead and the brothers became locked in a high-powered skirmish with police that saw hundreds of gunshots fired and several explosive devices detonated.

Tamerlan is dead, and the FBI is trying to piece together his profile. We know for sure the brothers have a Chechnyan background. Tamerlan spent six months in Dagestan, Russia, a city that borders Chechnya that the boys' father calls home.

Reports also reveal Tamerlan was a devout Muslim and that his religious views had grown ever-more extreme. He told his mother: "You know how Islam has changed me."

The change really took hold in 2011 when his mother encouraged him to turn to religion after the death of his friend.

"I told Tamerlan that we are Muslim, and we are not practicing our religion, and how can we call ourselves Muslims?" she told the WSJ.

"And that's how Tamerlan started reading about Islam, and he started praying, and he got more and more and more into his religion."

The FBI investigated Tamerlan that same year, concerned about his ties to Islamic extremists. In 2009, he was accused of assaulting his girlfriend.

But it was the escalating tension between Tamerlan's religious views and his family that drove a deeper wedge between them.

His parents had split up and Tamerlan's father had returned to Dagestan. Meanwhile in Boston, Tamerlan's behaviour was becoming increasingly erratic; he quit boxing despite a promising future; he encouraged his mother to cover herself up; and he was prone to frequent outbursts at a mosque in Cambridge, according to the report in the Wall St Journal.

Luis Vasquez, a former classmate of Tamerlan's, noticed a powerful change in his friend's attitude.

"I'm telling you, something turned," he said. "And it was dramatic."

'Have you come here to sleep?'

Tarmelan's parents insist he came to Dagestan and Chechnya last year to visit relatives and had nothing to do with the militants operating in the volatile part of Russia, with his father saying he slept a lot of the time.

However, the Boston bombing suspect couldn't have been immune to the attacks that savaged the region during his six-month stay.

The revelation comes as investigators reveal they believe Tamerlan and Dzhokhar Tsarnaev were likely planning other attacks based on the cache of weapons uncovered after Friday's dramatic manhunt in which Tamerlan was shot dead and Dzhokhar seriously wounded.

Investigators are now focusing on the trip that Tsarnaev made to Russia in January 2012 that has raised many questions.

His father said his son stayed with him in Makhachkala, the capital of Dagestan, where the family lived briefly before moving to the US a decade ago. The father had only recently returned.

"He was here, with me in Makhachkala," Anzor Tsarnaev told The Associated Press in a telephone interview.

"He slept until 3pm, and you know, I would ask him: 'Have you come here to sleep?' He used to go visiting, here and there. He would go to eat somewhere. Then he would come back and go to bed."

No evidence has emerged since to link Tamerlan Tsarnaev to militant groups in Russia's Caucasus.

On Sunday, the Caucasus Emirate, which Russia and the US consider a terrorist organisation, denied involvement in the Boston attack.

The Institute for Strategic Studies says the Caucasus Emirate is a radical Islamist group with ties to al-Qaeda and other international groups.

It aims to establish an Islamic state in the North Caucasus, but has gradually evolved as it was radicalised by an influx of Islamic extremism. While continuing and intensifying the separatist movement's attacks against the Russian state, the CE has also violently sought the implementation of Sharia law, engaged in sectarian violence and, in recent years, been implicated in various European terrorist plots.

Anzor Tsarnaev said they also travelled to neighbouring Chechnya.

"He went with me twice, to see my uncles and aunts. I have lots of them," the father said.

He said they also visited one of his daughters, who lives in the Chechen town of Urus-Martan with her husband. His son-in-law's brothers all work in the police force under Chechen leader Ramzan Kadyrov, he said.

Moscow has given Kadyrov a free hand to stabilise Chechnya following two wars between federal troops and Chechen separatists beginning in 1994, and his feared police and security forces have been accused of rampant rights abuses.

What began in Chechnya as a fight for independence has morphed into an Islamic insurgency that has spread throughout Russia's Caucasus, with the worst of the violence now in Dagestan.

In February, 2012, shortly after Tamerlan Tsarnaev's arrival in Dagestan, a four-day operation to wipe out several militant bands in Chechnya and Dagestan left 17 police and at least 20 militants dead.

In May, two car bombs shook Makhachkala, killing at least 13 people and wounding about 130 more. Other bombings and shootings targeting police and other officials took place nearly daily.

Tamerlan Tsarnaev's mother said he was questioned upon arrival at New York's airport.

"And he told me on the phone, 'imagine, mama, they were asking me such interesting questions as if I were some strange and scary man: Where did you go? What did you do there?'" Zubeidat Tsarnaeva recalled her son telling her at the time.

When the two ethnic Chechen suspects were identified, the FBI said it reviewed its records and found that in early 2011, a foreign government - which law enforcement officials confirmed was Russia - had asked for information about Tamerlan Tsarnaev. The FBI said it was told that Tsarnaev was a "follower of radical Islam" and was preparing to travel to this foreign country to join unspecified underground groups.

The FBI said that it responded by interviewing Tsarnaev and family members, but found no terrorism activity.

Both parents insist that the FBI continued to monitor Tamerlan Tsarnaev and that both of their sons were set up.