US Congressional intelligence leaders said authorities are pursuing "persons of interest" in the US in connection with the Boston Marathon bombings.
They asked for more help from Russian spy agencies.
Congressman Mike Rogers, the Republican chairman of the House Of Representatives intelligence committee, said better cooperation from Russia was needed in the probe of the two suspected bombers' recent contacts and activities.
Speaking on ABC's "This Week," Mr Rogers said he believed that Tamerlan Tsarnaev0, 26, clearly changed during his visit to Russia in 2012, becoming "radicalised."
"I think they [Russia] have information that would be incredibly helpful and that they haven't provided yet," he said.
New details of the unfolding investigation emerged following reports yesterday that Tamerlan Tsarnaev spoke to his mother about "jihad" in a 2011 phone call secretly recorded by Russian officials.
US authorities learned of the wiretapped discussion between Tamerlan Tsarnaev and his mother, Zubeidat Tsarnaeva, within the past few days, CBS News reported.
Tamerlan Tsarnaev died in an 18 April shootout with police, three days after the bombings that killed three people and injured more than 260 others.
Asked about the information Russia might have, Mr Rogers said of Russia's spy service: "The FSB is a hostile service to the FBI and the CIA and there is cultural problem there between where the Russians are and our folks.
"So they sent a letter, didn't have a lot of information."
Mr Rogers added that subsequent US requests for assistance have not been met.
"We still have persons of interest that we're working to find and identify and have conversations with," said Mr Rogers.
The Michigan lawmaker declined to say how many "persons of interest" there were.
"We are looking at phone calls before and after the bombing," the intelligence committee's senior Democrat, CA Dutch Ruppersberger of Maryland, also said on ABC.
The revelations that others are being sought for questioning came as the suspected bombers' father told Reuters today that he has abandoned plans to travel to the United States.
Speaking from a village in southern Russia, Anzor Tsarnaev said he believed he would not be able to see his surviving son, Dzhokhar, 19, who is being held in a federal prison in Massachusetts on charges he also carried out the bombing.
"In every investigation we have seen" when suspects carried out attacks, Mr Rogers told ABC, they had at least "affirmed" their plans to others.
In each case, Mr Rogers said, "there was outside affirmation of their intent to commit an act of jihad."
In the Boston bombing, "I believe that happened in the United States," he said.
Attention has turned to whether US officials missed signs that Tamerlan Tsarnaev may have posed a security threat, including a warning from Russia that he might be an Islamic militant.
Jihad can refer to a holy war waged on behalf of Islam as a religious duty, or to a Muslim's personal struggle in devotion to the faith.
The FBI interviewed Tamerlan Tsarnaev in 2011 but did not find enough cause to continue an investigation.
His name was on the US government's highly classified central database of people it views as potential threats, sources close to the bombing investigation have said.
Law enforcement authorities do not closely monitor the list, which includes about 500,000 people.