The college student suspected with his deceased older brother in the Boston Marathon bombing faces federal charges as he lay hospitalised under armed guard, severely wounded and unable to speak.
Dzhokhar Tsarnaev, 19, was captured with throat injuries that, coupled with sedatives administered at the Boston hospital where he is being treated, had left him incapable of speech and initially prevented authorities from questioning him.
"We're very anxious to talk to him and the investigators will be doing that as soon as possible," Ed Davis, Boston's police commissioner, said.
Tsarnaev's capture on Friday night ended a manhunt that virtually shut down greater Boston for some 20 hours. His older brother, Tamerlan Tsarnaev, 26, was pronounced dead after a gunfight with police a day earlier.
Investigators are seeking, among things, to determine whether the two suspects acted alone.
Boston's police commissioner and mayor have both said they believed the brothers were on their own.
"I am confident that they were the two major actors in the violence that occurred," Davis told CNN on Sunday.
Davis also said investigators have discovered at least four undetonated devices, one of them similar to the two pressure cooker bombs set off at the Boston Marathon, and that he believed the suspects were planning additional attacks.
Still, much of investigators' attention has focused on a trip to Russia last year by Tamerlan Tsarnaev.
The brothers emigrated to the United States a decade ago from Dagestan, a predominantly Muslim region in Russia's Caucasus.
They are accused of planting and setting off two homemade bombs near the crowded finish line of the Boston Marathon last Monday, killing three people and injuring more than 170 others.
Carmen Ortiz, US attorney and the federal prosecutor for the Boston area, was preparing criminal charges against the younger Tsarnaev, a naturalised US citizen, according to Davis. It was not clear when charges would be filed, but it could as early as Monday.
Trip to Russia
Tamerlan Tsarnaev traveled to Moscow in January 2012 and spent six months in Dagestan, a law enforcement source said.
Neighbours in Makhachkala, the region's capital city, said he kept a low profile while visiting there last summer, helping his father renovate an apartment unit.
That trip, combined with Russian interest in Tamerlan communicated to US authorities and an FBI interview of him in 2011, have raised questions whether danger signals were missed.
A group leading the armed struggle against Russia said on Sunday it was not at war with the United States, distancing itself from the Boston bombings. The conflict is rooted in two separatist wars that Russian troops waged against Chechen separatists following the fall of the Soviet Union.
Photos and video footage of the Tsarnaev brothers, allegedly in the act of planting bombs at the marathon, were first circulated by the FBI on Thursday with an appeal for help in locating the then-unidentified pair.
Police say the suspects shot dead a campus police officer at the Massachusetts Institute of Technology in Cambridge later that night, then hijacked a sport utility vehicle before opening fire and hurling explosives at pursuing law enforcement.
During this confrontation, according to police, a transit police officer was badly injured and the older Tsarnaev, walking toward officers and firing until he ran out of ammunition, was tackled by police, only to be struck by the SUV as his brother sped away in the vehicle.
The younger Tsarnaev later abandoned the vehicle and vanished, leading authorities to impose a lockdown on the city of Boston and surrounding communities before he was found and arrested in the suburb of Watertown the following night.
He turned up spattered with blood and hiding inside a covered boat parked in a back yard.
Students returning to campus on Sunday at the University of Massachusetts-Dartmouth, where Dzhokhar Tsarnaev was enrolled, recalled seeing him back in the dorm, at class and even working out in the gym a day or two after the bombings before realising he was suspected in the crime.
The brothers spent their early years in a small community of Chechens in the central Asian country of Kyrgyzstan, a mainly Muslim nation of 5.5mn. They moved in 2001 to Dagestan.
The men's parents, who moved back to southern Russia some time ago, have said their sons were framed.