Russia declined several FBI requests for more information on Boston Marathon bombing suspect Tamerlan Tsarnaev two years before the deadly 2013 attack.
The information, including a telephone call in which Tsarnaev and his mother discussed Islamic jihad, would probably have prompted harsher scrutiny of the suspect, The New York Times reported on Wednesday, citing an unpublished U.S. government review.
"They found that the Russians did not provide all the information that they had on him back then, and based on everything that was available, the FBI did all that it could," a senior U.S. official familiar with the review said.
The U.S. Department of Homeland Security did not immediately respond to requests for comment.
Two Chechen brothers, Tamerlan and his younger brother Dzhokhar, are suspected of planting pressure-cooker bombs near the race's finish line on Apr. 15, 2013, in an attack that killed three people and wounded more than 260.
Tamerlan died after a gunfight with police while the younger brother is awaiting trial on charges that could lead to the death penalty if he is convicted.
The new report, authored by the inspector general of the Office of Intelligence Community, has not been made public, though U.S. lawmakers are to be briefed on it on Thursday, the Times said.
This latest review comes after a March congressional report outlined what it called "missed opportunities" that could have prevented the attack.
That March report investigated the U.S. probe of Tamerlan Tsarnaev after a 2011 warning to the Federal Bureau of Investigation by Russian authorities that he had become radicalized and might return to Russia to join extremist groups there.
After the Russian warning, a task force of federal, state and local authorities launched an investigation that included checks of government databases and interviews with Tsarnaev and his parents. It found no evidence of terrorist activity.
But after an initial FBI probe in Boston, Russian officials refused several requests for additional information they had on Tsarnaev, although, at the time, U.S. law enforcement officials viewed him as a greater threat to Russia, the Times reported.
The new report found that Russians shared intelligence with the FBI only after the bombing attack, such as the telephone conversation about Islamic jihad.
"Had they known what the Russians knew they probably would have been able to do more under our investigative guidelines, but would they have uncovered the plot? That's very hard to say," a senior U.S. official said.
Boston-area FBI agents who investigated the Russian intelligence in 2011 could have conducted more interviews and should take steps to better share information with local and state agencies, the report said.