Tsarnaev , 26, and his 19-year-old brother, Dzhokhar Tsarnaev, are accused of setting off the two bombs near the finish line of the Boston Marathon on April 15 that killed three people and wounding more than 180 others.
Three days later, investigators say they killed a university police officer, carjacked a man and led police on a chase that resulted in a shootout that left Tamerlan Tsarnaev dead. His younger brother escaped, but was captured the next day, alive but badly wounded.
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.
No evidence has emerged since to link Tsarnaev to militant groups in Russia's Caucasus.
But a trip Tsarnaev made back to Russia in January, 2012, has raised questions.
His father said his son stayed with him in Makhachkala, the capital of Dagestan, where the family lived briefly before moving to the United States 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 3 p.m., 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."
He said his son went to the mosque for prayers, but would not have come under the influence of radical imams, who he said stay up in the mountain villages.
A woman who works in a small shop opposite Tsarnaev's apartment building said she only saw his son during the course of one month last summer. She described him as a dandy.
"He dressed in a very refined way," said Madina Abdullaeva. "His boots were the same color as his clothes. They were summer boots, light, with little holes punched in the leather."
Anzor Tsarnaev said they traveled together to neighboring 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 stabilize 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.
The Caucasus Emirate said Sunday that its mujahedin are not fighting with the United States. "We are at war with Russia, which is not only responsible for the occupation of the Caucasus, but also for heinous crimes against Muslims," it said in a statement on the Kavkaz Center website.
The group suggested that Russia's secret services would have had a greater interest in carrying out the attack in Boston.
Despite the violence in Dagestan, Anzor Tsarnaev said Sunday that his son did not want to leave and had thoughts on how he could go into business. But the father said he encouraged him to go back to the United States and try to get citizenship. Tamerlan Tsarnaev returned to the U.S. in July.
His mother said that 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.
Both parents insist that the FBI continued to monitor Tamerlan Tsarnaev and that both of their sons were set up.
Their mother went so far on Sunday to claim that the FBI had contacted her elder son after the deadly bombs exploded at the marathon. If true it would be the first indication that the FBI considered him a suspect before Boston descended into violence on Thursday.
At FBI headquarters in Washington, spokesman Michael Kortan stood by the bureau's public statement of two days ago in which the bureau described a 2011 FBI interview of Tamerlan Tsarnaev. Kortan said the 2011 interview was the only FBI contact with Tamerlan Tsarnaev. The FBI statement from two days ago says that the FBI did not learn of the identity of Tamerlan and his brother until Friday after the gun battle in which Tamerlan was killed.
The mother's claim could not be independently confirmed, and she has made statements in the past that appeared to show a lack of full understanding of what occurred in Boston.
Investigators released photos and video of the two Tsarnaev brothers on Thursday afternoon, but at that point their identities were not known. By late that night Tamerlan Tsarnaev was dead.
Tsarnaeva said her elder son told her by telephone that the FBI had called to inform him that they considered him a suspect and he should come in for questioning.
She said her son refused. "I told them, what do you suspect me of?" Tsarnaeva quoted her son as saying. "This is your problem and if you need me you should come to where I am."
He then told her he was going to drive his younger brother to the university, she said, speaking by telephone from Chechnya. Tsarnaeva claimed that her son later called his wife to tell her they were being chased and fired upon.
Associated Press writer Lynn Berry contributed from Moscow.