MOSCOW  - US lawmakers on a mission to Russia said on Sunday they had found no evidence that an American intelligence error enabled the Boston bombings, but that closer cooperation between Washington and Moscow might have helped to thwart the attack.

US investigators suspect two brothers who emigrated from Russia, one since shot dead by police, staged the attacks at the Boston Marathon in April when three people died and 264 were injured.

Two congressmen on the fact-finding visit to Russia said the two countries - former Cold War foes now at odds over issues from Syria to President Vladimir Putin's treatment of opponents - had to work together better against a shared threat from Islamist militants.

"Radical Islam is at our throat in the United States, and it is at the throat of the Russian people," said Republican US Representative Dana Rohrabacher, who led a group of six US lawmakers on the weeklong visit to Russia.

President Barack Obama's administration and US intelligence have faced scrutiny over claims they failed to see the danger from the suspects Tamerlan and Dzhokhar Tsarnaev, ethnic Chechens who emigrated with their parents a decade ago.

"We've been asked a number of times, do we believe that the Boston Marathon massacre could have been thwarted - could it have been prevented? And the answer is, there's nothing specific that could have been done that we can point to that, had it been done differently, would have prevented this," Rohrabacher said.

"But we can say that had we had a much higher level of cooperation all along, so that the whole situation would have been different, I believe that would have been one of the type of things we could have thwarted," he told a news conference at the US Embassy in Moscow.

On their visit, the lawmakers met Federal Security Service (FSB) officials and visited the North Caucasus town of Beslan, scene of a deadly 2004 school siege some Russians call their country's equivalent of the September 11, 2001 attacks on the United States.

US officials have said Russian security services asked the FBI about Tamerlan Tsarnaev in early 2011 out of concern he had embraced radical Islam and would travel to Russia to join insurgents.

FBI agents interviewed him in Massachusetts in 2011 but said they found no serious reason for alarm. US officials say Russia's FSB security services later failed to respond to the FBI's requests for more information about him.

Reading from notes from a briefing with FSB officials, Republican US Representative Steve King said they indicated the FSB had told the FBI that Tamerlan was "very close to radical Islam and very religious".

"I suspect that he was raised to do what he did," King said of Tamerlan Tsarnaev, 26, who died in an April 19 shootout with police. Dzhokhar, 19, is in a Massachusetts prison hospital awaiting trial on charges that can carry the death penalty.

Tamerlan Tsarnaev travelled to Russia early in 2012 and spent six months in Dagestan, a North Caucasus province that is now at the centre of the Islamist insurgency rooted in two post-Soviet separatist wars in neighbouring Chechnya.

Rohrabacher, asked whether US authorities reacted appropriately to the information conveyed by Russia, said: "I think that given the circumstances and the level of cooperation, I would say they did - but I would also say that the level of cooperation was unacceptable."

The Kremlin has called for closer intelligence cooperation after the Boston bombings and high-level meetings have been held, but Russia's expulsion of an alleged US spy recruiter last month underscored persistent tension.