By Svea Herbst-Bayliss and Ben Berkowitz
BOSTON (Reuters) - Federal prosecutors prepared criminal charges on Sunday against the 19-year-old suspect in the Boston Marathon bombings who was seriously wounded, unable to speak, and under heavy guard at a city hospital two days after his dramatic capture.
Investigators could not interview Dzhokhar Tsarnaev as they searched for clues to what may have driven him and his brother, ethnic Chechens who came to the United States 10 years ago, to plant the bombs, and whether anyone else was involved.
Receiving particular scrutiny was a six-month trip the brother, Tamerlan Tsarnaev, took to Russia in January 2012, and whether he was influenced by Chechen separatists or the Islamist struggle against the West.
Tamerlan Tsarnaev, 26, died after a Thursday night shootout with police that left behind some 250 spent shells on the streets of Watertown, the Boston suburb where police finally cornered the younger Tsarnaev after a massive manhunt that shut down greater Boston on Friday.
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Dzhokhar Tsarnaev, a naturalized U.S. citizen, was found that night, covered in blood and hiding on a boat in a Watertown back yard.
U.S. Attorney Carmen Ortiz, the federal prosecutor for the Boston area, was preparing criminal charges, according to Boston Police Commissioner Ed Davis. It was not clear when charges would be filed. Prosecutors did not plan any news conference or announcements on Sunday.
The suspect was watched by armed guards in the intensive care unit of Beth Israel Deaconess Medical Center, where his brother was pronounced dead early on Friday.
"He is not in a condition to be interrogated at this point in time," Davis said in an afternoon news conference.
Davis said earlier police discovered at least four unexploded devices including one similar to the two pressure cooker bombs packed with nails and ball bearing used in the twin blasts Monday that killed three people and wounded more than 170 near the finish line of the Boston Marathon.
"I personally believe they were (planning other attacks)," he told CBS television's "Face the Nation."
The men's parents, who moved back to southern Russia, have said their sons were framed.
Tsarnaev was shot in the throat, U.S. Senator Dan Coats, a member of the Intelligence Committee, told ABC. A source close to the investigation told Reuters he had tongue damage.
"We don't know if we'll ever be able to question the individual," Boston Mayor Thomas Menino said Sunday, also on ABC's "This Week" program.
Runners in the London Marathon held 30 seconds of silence before starting their race on Sunday, while people from the greater Boston area that had been on virtual lockdown all day Friday remembered the victims in church services.
"We must be people of reconciliation and not revenge," Roman Catholic Cardinal Sean O'Malley told a packed Cathedral of the Holy Cross. "The crimes of the two young men must not be justification for violence against Muslims."
In the neighboring city of Cambridge, police stationed themselves across from a home where various members of the Tsarnaev family had lived, advising gawkers and bystanders to move on.
Patricia McMillan, who lives two doors down, said she last saw Tamerlan Tsarnaev in the neighborhood the Wednesday before the bombing, noting he had shaved off his beard and that he was smoking.
TRIP TO RUSSIA
Tamerlan Tsarnaev traveled to Moscow in January 2012 and spent six months in the region, a law enforcement source said.
That trip, combined with Russian interest in Tamerlan communicated to U.S. authorities and an FBI interview of him in 2011, have raised questions whether danger signals were missed.
It was unclear if he could have had contact with militant Islamist groups in southern Russia's restive Caucasus region.
A group leading an Islamist insurgency against Russia said on Sunday it was not at war with the United States, distancing itself from the Boston bombings.
"We are fighting with Russia, which is responsible not only for the occupation of the Caucasus but for monstrous crimes against Muslims," said a statement from Caucasus Emirate militants operating in Dagestan.
The insurgency is rooted in two separatist wars that Russian troops waged against Chechen separatists following the fall of the Soviet Union.
The family emigrated to the United States about a decade ago. The brothers spent their early years in a small community of Chechens in the central Asian country of Kyrgyzstan, a mainly Muslim nation of 5.5 million. They moved in 2001 to Dagestan, a southern Russian province where their parents now live.
Neighbors said they noticed nothing unusual about Tsarnaev, who this summer helped his father renovate his first floor apartment in Makhachkala, a bustling city in Dagestan.
"They say he was a fanatic. I didn't see that," said Madina Abdulayeva, 45, who runs the small grocery shop across the pot-holed street where he used to come to chat. "We're all Muslim here. We're all part of Islam. We all pray.
(Additional reporting by Mark Hosenball in Washington; Writing by Daniel Trotta; Editing by Vicki Allen and Doina Chiacu)