(WASHINGTON) -- Intelligence community watchdogs are extending their review of what the U.S. government knew beforehand about two brothers accused of carrying out the deadly April 15 Boston Marathon bombings, according to a government letter obtained by ABC News.
The joint letter from four inspectors general addressed to congressional committees that oversee national security, dated Friday, said their final report on what intel agencies knew about Tamerlan and Dzhokhar Tsarnaev before they allegedly used two bombs to kill three people and injure more than 260 others won't be finished this month, as anticipated.
"Information relevant to the review is still being provided to the review teams," the letter reads.
The coordinated review inside the Obama administration is being led by Charles McCullough, the Intelligence Community Inspector General, working with the IGs from the CIA, Department of Homeland Security and Department of Justice.
"The inspectors general have been working diligently toward completing this review," reads the letter from the Intelligence Community Inspectors General Forum. But no completion date was offered. "When all requested information has been received and analyzed, we will be able to provide a better estimate of the timeframe for producing a draft report."
The Office of the Director of National Intelligence declined to say if the review team has faced any resistance from agencies over access to files, witnesses or other information, but one U.S. official insisted to ABC News that there "hasn't been any intransigence" or stonewalling.
The House Committee on Homeland Security is investigating the Boston bombings and the Senate Homeland Security and Governmental Affairs Committee also has held hearings.
Tamerlan Tsarnaev, who was killed in a shootout with police days after the April bombings, was flagged by the Russian government in 2011 for possible extremism, the FBI has said. U.S. counter-terrorism agents subsequently examined the young immigrant from Dagestan and Chechnya -- including interviews with Tamerlan Tsarnaev and his family -- before closing the inquiry.
The CIA also requested that U.S. agencies put the older Tsarnaev brother on a watchlist, despite finding no "derogatory" information about him. Requests to the Russian government for details about their suspicions that Tamerlan Tsarnaev was a radicalized Islamist extremist were repeatedly ignored, U.S. officials have said.
He was able to travel to Dagestan and Chechnya for several months last year, where his activities have never fully been detailed or accounted for. Former FBI Director Robert Mueller recently told ABC News that the brothers did not have ties to foreign extremists, however.
Tamerlan's younger brother, Dzhokhar, was injured in a police shootout and captured days after the bombing. He has pleaded not guilty to charges against him, including murder, in connection with the crime.
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