By Richard Valdmanis

BOSTON (Reuters) - 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 by 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, 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 transformed into 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 before his six-month trip to Russia's Dagestan region.

Afterward, Franca said he saw Tamerlan in Boston and he appeared "different" - he had a beard, dressed in white, and his wife was covered and submissive.

"He asked me, you are not Muslim yet?" Franca said. "I never expected he could ask me such a thing."

Another man, Brandon Douglas, who trained with Tamerlan at a gym in Brighton, said he also noticed changes. "His dress changed. The beard obviously was a pretty drastic departure."

Douglas said he previously knew Tamerlan as "a very flashy dresser. Shiny alligator shoes and things of that nature."

Defense attorneys opened their case Monday arguing 26-year-old 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 a computer registered to Tamerlan’s widow, Katherine Russell, showed Internet searches in 2012 that included the words "Rewards for wife of mujahideen."

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

(Additional reporting by Scott Malone; Editing by Leslie Adler and Ted Botha)