Marathon bombing suspect Dzhokhar Tsarnaev remained in serious condition in a Boston hospital on Sunday unable to answer questions on the devastating attack, officials said.

Investigators released stunning images of Tsarnaev's final moments of freedom, slumbering wounded in a boat in a suburban backyard. They are also stepping up inquiries into a trip to Muslim regions of Russia taken by his accomplice brother, 26-year-old Tamerlan Tsarnaev.

The 19-year-old is "serious but stable," Massachusetts governor Deval Patrick told reporters Saturday. "I think not able to communicate yet."

US media said he suffered a throat wound during his cavalcade which ended late Friday after a massive manhunt in which Tamerlan was killed.

Despite the throat wound Tsarnaev cursed profusely in the ambulance ride to the hospital after his arrest, the Los Angeles Times reported Sunday, citing an FBI source.

The Tsarnaev brothers are the main suspects in the double bomb attack on the Boston marathon which killed three people wounded about 180. A policeman was killed and another was left fighting for his life after gunbattles during the hunt.

Governor Patrick said he hopes the teenager survives. "We have a million questions and those questions need to be answered," he added.

Dzhokhar Tsarnaev was under armed guard at a hospital where some victims of the bombings are also being treated. Counter-terrorism agents trained in interrogating "high-value" detainees were waiting to question him, a law official told AFP.

Prosecutors were also at the Beth Israel Deaconess Hospital in Cambridge, just outside Boston, working out charges.

Media reports say authorities did not read Tsarnaev his normal rights to see a lawyer or stay when he was captured, invoking a special exception for security reasons.

That has left US authorities facing tough decisions over how to handle the investigation and any trial.

Republican Senators John McCain and Lindsey Graham have led calls for the teenager to be declared an "enemy combatant," which would give him the same status as Guantanamo "war on terror" detainees.

Legal rights groups have been quick to insist that he face a criminal trial, even though Tsarnaev would be likely to face a death penalty calls.

The Tsarnaev family are ethnic Chechens who moved to the United States from the former Soviet state of Kyrgyzstan around 2002.

Media reports say Tamerlan Tsarnaev became a fervent Muslim in recent years. Much focus is now being put on his six month trip to the Russian region of Dagestan last year.

US President Barack Obama and Russian President Vladimir Putin agreed during telephone talks Saturday to increase cooperation against international terrorism, the Kremlin and White House said.

The FBI acknowledged on Friday that an unnamed foreign government, reportedly Russia, asked for information about Tamerlan Tsarnaev in 2011. The FBI interviewed the man but said it had found no "derogatory" information.

The men's social media pages appeared to express sympathy with the struggle in Chechnya, which has been ravaged by two wars since 1994 between Russia and Islamist-leaning separatist rebels.

Dzhokhar Tsarnaev, who became a US citizen last year, was caught after a man named Dave Henneberry saw blood on boat he kept in his backyard in Watertown, in the Boston suburbs. When Henneberry lifted the tarp he saw the wounded teenager curled up inside, police said.

The University of Massachusetts student was surrounded for a showdown that included a final gunbattle before Tsarnaev surrendered to authorities.

Thermal images taken by a police helicopter overhead showed Tsarnaev slumbering in the covered boat. They also showed a robotic arm reaching to lift the cover so cameras could peek inside.

Over the weekend crowds gathered outside Henneberry's home, craning their necks to gaze at his bullet-riddled and blood-smeared 1980s boat.

Neighbor George Pizzuto said that Henneberry was "in shock" and "totally distraught" at the events of Friday night, and is not talking to reporters.

Henneberry has gone into hiding and the boat, his prized possession, is being kept under wraps by police as they collect evidence against Tsarnaev.

Police say the brothers killed one officer and wounded another as they fled, hurling home-made bombs at their hunters.

Tamerlan Tsarnaev died in one gunbattle. His brother was also hit but escaped.

Watertown police chief Edward Deveau said the pair had at least six bombs with them when being chased and that Dzhokhar had driven over his brother as he escaped.

Fifty-eight of the victims from the bomb attack are still in Boston hospitals, with three in critical condition.