WASHINGTON (AFP) - A US Senator said on Monday that the FBI and Russian intelligence may have missed warning signs and made basic errors like misspelling a terror suspect's name in the run up to the Boston bombings.

Republican Lindsey Graham said an associate director of the Federal Bureau of Investigation briefed him late Sunday that Russian authorities did alert the FBI that terror suspect Tamerlan Tsarnaev, an ethnic Chechen and legal US resident, "had become radicalized" and intended to join underground groups.

But after the FBI conducted extensive investigations on the suspect, including interviews with relatives and classmates, and wrote Moscow to request more data, the case went cold, according to Mr Graham.

"According to the FBI, the Russians never responded," Mr Graham told reporters, as US authorities began to dig deeper into the two brothers who they believe are at the centre of last week's twin bombings at the finish of the Boston Marathon that killed three people and injured 200.

Tamerlan Tsarnaev, 26, was killed in a shootout with police overnight on Thursday, while his 19-year-old brother Dzhokhar was captured on Friday and is being held on terror charges.

Mr Graham said that, despite being placed on a US government watch list, the older Tsarnaev was able to fly undetected to Russia.

"I've been told by the FBI that the reason that his name did not pop up in the system was because it was misspelled," Mr Graham said.

"We don't know if he misspelled it," or if it was the fault of the airline, identified by Mr Graham as Russian flagship Aeroflot. "We certainly missed it here," Mr Graham said.

Some US lawmakers allege it was Tamerlan's trip to Russia that ultimately led to last week's attack, and questioned why the FBI did not pay more attention to the suspect after his return, including his online postings of extremist material.

"How could we have missed all the activity on the Internet and YouTube, given the fact he had already been identified?" Mr Graham asked.

The senator, who openly criticised the White House's decision not to treat Dzhokhar Tsarnaev as an "enemy combatant," called for congressional hearings that review US law outlining how citizens or legal residents under surveillance can be treated.

"The FBI was telling me they were concerned about this individual, but there were limits on their ability to track what he did."