BOSTON — The older brother of Boston Marathon bomber Dzhokhar Tsarnaev changed from a roisterous boxer known for flashy outfits into someone with a bushy beard and drab clothes and obsessed with militant Islam, following a trip to Russia in 2012, witnesses said on Tuesday.

The testimony before a federal jury in Boston comes as lawyers for Tsarnaev build their argument that the 21-year-old ethnic Chechen was a pawn in his now-dead brother’s scheme to bomb the race on April 15, 2013, and should be sentenced to life in prison, not death.

Tsarnaev was convicted this month of killing three people and injuring 264 in the bombing; and shooting dead a police officer three days later alongside the older brother, Tamerlan, 26, a 2009 New England boxing champion.

Rogerio Franca, who lived near the Tsarnaevs in Cambridge, Massachusetts, told jurors Tamerlan was a partier before his trip to Russia, but became someone more closely resembling a devout Muslim when he returned.

“Most of the time he was drunk; most of the time he was high,” Franca said of Tamerlan. After his return from Russia’s Dagestan region, Franca said he saw  that Tamerlan appeared “different” – he had a beard, dressed in white and his wife was covered and submissive.

Jurors later heard transcripts of FBI interviews with two of Tamerlan’s acquaintances who claimed Tamerlan appeared to have been radicalized and vocally supported violent jihad.

One of the men, Magomed Dolakov, said he had had an argument with Tamerlan shortly before the marathon bombing in which Tamerlan defended the bombing of a policeman’s funeral in Chechnya because he was not Muslim, according to transcripts read to the jury.

Defense attorneys opened their case on Monday arguing that Tamerlan, who died following a gunfight with police days after the bombing, was the driving force behind the attack.

Prosecutors previously cited al Qaeda materials found on Dzhokhar’s computers and a note he wrote suggesting that the attack was retribution for U.S. military campaigns in Muslim countries.

Digital forensics expert Mark Spencer testified on Tuesday that Tamerlan’s laptop contained graphic videos and images of dead and injured civilians in Syria.

Martin Richard, 8, Chinese exchange student Lu Lingzi, 23, and restaurant manager Krystle Campbell, 29, died in the Boston bombing. Richard’s parents and Collier’s sister have asked prosecutors to drop their pursuit of the death sentence.