Christina DiIorio Sterling, a spokeswoman for U.S. Attorney Carmen Ortiz, said in an email late Wednesday, "This remains an ongoing investigation and we don't have any further comment."
Investigators have said the brothers appeared to have been radicalized through jihadist materials on the Internet and have found no evidence tying them to a terrorist group.
U.S. investigators traveled to the predominantly Muslim province of Dagestan in Russia and were in contact with the brothers' parents, hoping to gain more information.
They are looking into whether Tamerlan, who spent six months in Russia's turbulent Caucasus region in 2012, was influenced by the religious extremists who have waged an insurgency against Russian forces in the area for years. The brothers have roots in Dagestan and neighboring Chechnya but had lived in the U.S. for about a decade.
Dzhokhar told the FBI that they were angry about the U.S. wars in Afghanistan and Iraq and the killing of Muslims there, officials said.
Dzhokhar's public defender had no comment on the matter Wednesday. His father has called him a "true angel," and an aunt has insisted he's not guilty.
Investigators have found pieces of remote-control equipment among the debris and were analyzing them, officials said. One official described the detonator as "close-controlled," meaning it had to be triggered within several blocks of the bombs.
That evidence could be key to the court case. And an FBI affidavit said one of the brothers told a carjacking victim during their getaway attempt, "Did you hear about the Boston explosion? I did that."
Officials also recovered a 9 mm handgun believed to have been used by Tamerlan from the site of an April 18 gunbattle that injured a Massachusetts Bay Transportation Authority officer, two U.S. officials said.
In other developments:
— Vice President Joe Biden condemned the bombing suspects as "two twisted, perverted, cowardly, knockoff jihadis" while speaking at a memorial service Wednesday for Sean Collier, a campus police officer at the Massachusetts Institute of Technology who was ambushed in his cruiser three days after the bombing. More than 4,000 mourners paid tribute to the officer.
— The Office of Health and Human Services in Massachusetts confirmed a Boston Herald report Wednesday that Tamerlan, his wife and toddler daughter had received welfare benefits up until last year, when he became ineligible based on family income. The state also says Tamerlan and his brother received welfare benefits as children through their parents while the family lived in Massachusetts.
— The area around the marathon finish line was reopened to the public.
Yost and Jakes reported from Washington. Associated Press writers Kimberly Dozier, Matt Apuzzo, Eileen Sullivan, Adam Goldman and Eric Tucker in Washington, David Crary, Denise Lavoie, Bridget Murphy and Bob Salsberg in Boston and Lynn Berry in Moscow contributed to this report.
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