The Mayor of Boston, Tom Menino, has said authorities don't know if they will ever be able to question the marathon bombing suspect.
Dzhokhar Tsarnaev, 19, remains in a "very serious" condition at a Boston hospital following his captured after eluding police for 24 hours in a bloody manhunt.
Mr Menino told ABC's "This Week": "We don't know if we'll ever be able to question the individual."
Yesterday, Massachusetts Gov. Deval Patrick said that Tsarnaev is apparently unable to communicate at the moment.
There was no immediate word on when Tsarnaev might be charged and what those charges would be.
The twin bombings killed three people and wounded more than 180.
Tsarnaev's brother, Tamerlan, 26, was killed in a firefight with police earlier on Friday as officers hunted them for the twin blasts on Monday that killed three and injured 176.
US Senator Dan Coats, a member of the Intelligence Committee, said it was questionable whether Tsarnaev would be able to talk again.
"The information that we have is that there was a shot to the throat," Coats told the ABC program.
"It doesn't mean he can't communicate, but right now I think he's in a condition where we can't get any information from him at all," said Coats, an Indiana Republican.
In London the marathon, which went ahead despite concerns after events in Boston, appeared to pass without incident. Thousands of runners offered tributes to those killed and injured in Boston.
The race began after a moment of silence for the victims, and many here wore black armbands as a sign of solidarity. Two runners finished carrying a banner that read "For Boston."
"It means that runners are stronger than bombers," said Valerie Bloomfield, a 40-year-old participant from France.
London's is the first major international marathon since two bombs exploded near the finish line in Boston. The blasts killed three people and wounded 180, and a policeman died during the search for the bombers. One suspect died in a shootout with police, while a second was caught.
Some 35,000 runners took part in the London race, which also drew tens of thousands of spectators. Earlier, authorities said they were boosting the police presence by 40 percent and adding extra surveillance as precautionary measures.
Today, questions were being asked about the actions of the FBI following the revelation that the FBI received a tip from a foreign government that Boston bombings suspect Tamerlan Tsarnaev was a radicalised Islamicist.
The information, which was reportedly received two years ago but was not deemed worthy of further attention.
An FBI statement said the request "was based on information that he was a follower of radical Islam and a strong believer, and that he had changed drastically since 2010 as he prepared to leave the United States for travel to the country's region to join unspecified underground groups".
It is believed that the request was made by Russia.