Boston Marathon bomb suspect number two is in custody after he was arrested after a brief stand-off. Fox News

UPDATE: BOSTON Marathon bombing suspect Dzhokhar Tsarnaev is awake and responding sporadically in writing to questions, according to unconfirmed reports.

Several news outlets are reporting that Tsarnaev, 19, was responding sporadically in writing to questions from police investigators.

The report has not been veriifed by authorities.

Investigators were reportedly asking about other cell members and other unexploded bombs, law enforcement sources told ABC News.

The report was picked up by CNN's Piers Morgan, but veteran Al-Jazeera correspondent Alan Fisher cautioned that Tsarnaev was likely too ill to be reponsive.

NBC News, citing federal officials, reported that despite a throat injury that keeps him from talking, the 19-year-old was beginning to respond to questions from investigators

USA Today also cited a law enforcement official as saying he was awake and responding in writing.

Tsarnaev is being treated in Beth Israel Medical Center in Boston where he is listed as being in a “critical but stable condition” from wounds sustained in a shootout, police say.

Investigators believe his neck wound may be from a failed suicide attempt and not from the shoot-out that killed his brother, CBS News and The New York Times report.

The wound has "the appearance of a close range, self-inflicted style," The New York Times quotes a senior law enforcement official saying. "He's not in good shape."

The injury is why he is unable to communicate with police, CBS reports.

Tsarnaev was taken into custody bloodied and badly wounded after an hours-long manhunt and an earlier shootout that killed his older brother Tamerlan, 26.

“The suspect is not yet able to be interrogated by police,” said Boston Police Department Commissioner Ed Davis.

Boston Mayor Thomas Menino added “we don't know if we'll ever be able to question the individual”.

Officials have invoked a “public safety” legal exception that will allow them to question Tsarnaev without reading him his so-called Miranda rights to remain silent and to consult a lawyer.

Reports said charges could come as early as today against Tsarnaev, who with his brother is the main suspect in the double bomb attack on the Boston marathon that took three people's lives and wounded about 180.

A policeman was killed and another was seriously wounded in a shootout with the suspects.

Authorities have yet to disclose the exact nature of Tsarnaev's injuries as he receives treatment at Beth Israel Deaconess Medical Centre, where some of thse wounded in the blasts are also being treated.

Commissioner Davis said federal authorities were trying to track down how and where the two suspects obtained the firearms and explosive devices.

Some Republican lawmakers have called for declaring the teenager an “enemy combatant,” which would give him the same status as Guantanamo “war on terror” detainees.

Critics have insisted that because Tsarnaev is a naturalised US citizen and authorities have found no ties between him and terror groups so far, he should be granted a criminal civilian trial.

With Dzhokhar Tsarnaev unable to speak, attention has shifted to Tamerlan, who may have been radicalised or even trained in the Caucasus last year.

US lawmakers questioned why Tamerlan Tsarnaev, killed in a shootout, did not raise more red flags despite being questioned at the request of the Russian government in 2011 and spending six months in the volatile region last year.

Earlier, 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.

NBC News reported that authorities were finalising federal charges against Tsarnaev and hoped to charge him later today.

"He is in no condition to be interrogated at this time," Boston Police Commissioner Ed Davis told Fox News Sunday.

"He's progressing, though, and we're monitoring the situation at this time."

Mr Davis said a specialist interrogation team is waiting to question the suspect.

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

Mr Patrick said he hopes the teenager survives.

"We have a million questions and those questions need to be answered," he said.

Overnight, family and friends attended a wake at a funeral home in Medford, Massachusetts, for Krystle Campbell, the 29-year-old restaurant worker who was one one of the three people killed in the marathon bombing. A private funeral is scheduled for today.

Eight-year-old Martin Richard of Boston's Dorchester neighbourhood and 23-year-old Lu Lingzi, a Boston University graduate student from China, also died in the attacks. Boston University is holding a memorial service for Lu today.

Surveillance video from the Boston Marathon attack shows one suspect dropping his backpack and calmly walking away from it before the bomb inside it exploded, Massachusetts Gov. Deval Patrick said Sunday.

The video clearly puts 19-year-old Dzhokhar Tsarnaev at the scene of the attack, Mr Patrick said on NBC.

"It does seem to be pretty clear that this suspect took the backpack off, put it down, did not react when the first explosion went off and then moved away from the backpack in time for the second explosion,'' said Mr Patrick.

"It's pretty clear about his involvement and pretty chilling, frankly.''

US media reported Tsarnaev suffered a throat wound during a shoot-out with police 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, 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.

Commissioner Davis said authorities found an arsenal of homemade explosives after Friday's gun battle between police and the two suspects.

"We have reason to believe, based upon the evidence that was found at that scene - the explosions, the explosive ordnance that was unexploded and the firepower that they had - that they were going to attack other individuals," Nr Davis told CBS television.

"That's my belief at this point."

The scene of the gun battle was loaded with unexploded bombs, and authorities had to alert arriving officers to them and clear the scene, Mr Davis said. One improvised explosive device was found in the Mercedes the brothers are accused of carjacking, he said.

"This was as dangerous as it gets in urban policing," Mr Davis said.

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 usual Miranda 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 US 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 Mr Henneberry's home, craning their necks to gaze at his bullet-riddled and blood-smeared 1980s boat.

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

Mr 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 shot but escaped, bloody and wounded.

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.

EARLIER

Police Commissioner Davis said releasing photos was a turning point for the manhunt, as the wait to interview a suspect continues.

Mr Davis has told the Boston Globe that releasing the photos of the two suspects in the Marathon bombing “may have led to the further attack” against MIT police officer Sean Collier.



He said it "was a turning point in the investigation, no doubt about it".



"It forced them out of their hideout and they decided to commit further violent acts."

His comments came as residents of Watertown, Boston, gathered to pay tribute to the the victims of the Boston Marathon bombing, after they were forced in to lockdown mode until the manhunt was over.

Investigators have also examined the boat where Dzhokhar Tsarnaev, 19, was hiding and shot multiple times before he was taken into police custody.

They have also searched the house of their sister, Alina, Tsarnaev, who lives in the US.

The FBI have seized a computer from her home and he is cooperating with police.

Their motives for committing such a crime are still unknown.

Late Friday the FBI confirmed it had investigated Tamerlan Tsarnaev two years ago. The FBI said it received information from "a foreign government" that he was a follower of radical Islam and had changed drastically in 2010 as he prepared to travel overseas to join "unspecified underground groups".

An uncle of the Tsarnaev brothers said he had a falling-out with Tamerlan over the man's increased commitment to Islam.

Ruslan Tsarni said Tamerlan told him in a 2009 phone conversation he had chosen "God's business" over work or school. Tsarni said he then contacted a family friend who told him Tsarnaev had been influenced by a recent convert to Islam.

Tsarni said his relationship with his nephew basically ended after that call.

As for Dzhokhar Tsarnaev, "he's been absolutely wasted by his older brother. I mean, he used him. He used him for whatever he's done," Tsarni said.

Tamerlan Tsarnaev studied accounting as a part-time student for three semesters from 2006 to 2008 and was married with a young daughter.

Dzhokhar Tsarnaev was a student at the University of Massachusetts-Dartmouth.