Federal intelligence officials are looking at whether more could have been done to prevent the Boston Marathon attacks, President Barack Obama said Tuesday, though he added that he's not aware of any missteps.
"Based on what I can see so far, the FBI performed its duties. The Department of Homeland Security did what it was supposed to be doing," Obama said. "But this is hard stuff."
The president called the review by the Director of National Intelligence's office "standard procedure," but it comes amid withering criticism from some lawmakers of how well law enforcement, intelligence analysts and the administration handled a 2011 request by Russian officials to investigate one of the two bombing suspects, Tamerlan Tsarnaev.
That year, Russian authorities alerted the United States to concerns that Tsarnaev was becoming increasingly radical. The Russians also raised questions about Tsarnaev's mother, Zubeidat Tsarnaev, according to several sources.
But the FBI found no evidence of extremist activity and closed the case. The names of both Tsarnaev and his mother were placed in a terror database, however.
Still, Tsarnaev was allowed to travel the next year to a restive Russian region rife with Islamist terror groups, and he returned to the United States after six mysterious months abroad.
Investigators have said they are looking at possible links between Tsarnaev and those groups during his time in the region.
In the days following the attacks, Republican Sen. Lindsey Graham of South Carolina led criticism of the administration's handling of the Russian reports -- questioning whether intelligence and law enforcement agencies properly shared information that could have prevented the April 15 bombings.
Three people died in the attack and more than 260 were wounded, 20 of whom remained hospitalized Tuesday. Authorities say Tamerlan Tsarnaev and his younger brother, Dzhokhar, killed a police officer a few days after the attack.
"I just find it really unnerving that we could have had him in FBI custody in 2011 and did a whole profile of him, and after the attack that his name did not surface, that we didn't check the database or the database had him missing," Graham had said of the older Tsarnaev.
He continued his criticism Tuesday, after Obama's remarks, comparing the Boston attack with one last year at the U.S. consulate in Benghazi, Libya, which left four Americans dead. Both attacks, he said, suggesting warning signs were ignored and communication among intelligence and law enforcement agencies was flawed.
Obama said the intelligence review into how Tsarnaev's case was handled, while not prompted by the criticisms, would "leave no stone unturned."
"We want to see, is there in fact additional protocols and procedures that could be put in place that would further improve and enhance our ability to detect a potential attack," he said.
At the same time, the president rejected Graham's criticisms, saying "it's not as if the FBI did nothing."
"They not only investigated the older brother, they interviewed the older brother," the president said. "They concluded that there were no signs that he was engaging in extremist activity."
Tamerlan Tsarnaev's body, meanwhile, remained Tuesday in the hands of the Massachusetts Medical Examiner's Office.
His widow, Katherine Russell, released a statement Tuesday evening stating that the medical examiner's office "is prepared to release" the slain suspect's remains. If and when they do, it won't be to Russell, the mother of his young child.
"It is Katherine Russell's wish that his remains be released to the Tsarnaev family, and we will communicate her wishes to the proper authorities," her lawyers said in a statement.
Through her attorneys, Russell has denied any knowledge of her husband's involvement in the bombings. She will meet in the coming days with law enforcement, "as she has done for many hours over the past week, and provide as much assistance to the investigation as she can," the lawyers said.
The FBI took DNA samples at her Rhode Island home on Monday -- the same day law enforcement sources told CNN that a woman's DNA had been found on a fragment of one of the pressure cooker bombs used in the bombings.
The sources cautioned that this doesn't necessarily mean a woman might have conspired with brothers. And an official said that, even if Russell's DNA matches that from the bomb fragment, it doesn't necessarily mean she participated in the bomb's construction.
The DNA could also be from a victim, Lawrence Kobilinsky, a DNA expert at the John Jay College of Criminal Justice in New York, told CNN's Erin Burnett.
As of Tuesday, there was no known match for the DNA found on the bomb fragment. Nor was there a match from at least one fingerprint found in the Boston bomb debris, a law enforcement official with knowledge of the investigation told CNN's Susan Candiotti.