Site of Boston attack
Site of Boston attack AFP photo

Boston Marathon bombing suspect Dzhokhar Tsarnaev is awake and responding sporadically in writing to questions, authorities said on Sunday, according to ABC News.

Investigators are asking about other cell members and other unexploded bombs, the report said.

Previously officials said Tsarnaev was in no condition to be interrogated.

Tsarnaev, 19, is being treated at Beth Israel Deaconess Medical Center in Boston, where he is listed in serious but stable condition, with wounds to the neck and throat area, according to sources.

The bombing killed three, including an eight-year-old boy, and wounded about 170. An MIT officer was allegedly killed by Tsarnaev and his brother Tamerlan and a Boston transit cop was badly wounded in a subsequent shootout. Tamerlan died during the shootout.

Dzhokhar was caught by the authorities on Friday night after an exchange of gunfire at the Boston suburb of Watertown.

Boston Mayor Thomas Menino said on Sunday  he hopes federal authorities "throw the book at him," which would include the possibility of execution, according to ABC News.

Tsarnaev could face charges at the state and federal levels, but Massachusetts has no death penalty.

Menino said he believed the Tsarnaev brothers acted alone when they allegedly set off two bombs at the Boston Marathon finish line.

He also said he thought that Tamerlan Tsarnaev, 26, "brainwashed or manipulated" his 19-year-old brother.

British tabloid newspaper The Mirror reported on Sunday that police believe the Tsarnaev brothers were specially trained to carry out the devastating attack, and were part of a larger "sleeper cell." This report is not corroborated by other news outlets and its veracity cannot be ascertained.

The Mirror quoted a source close to the investigation who said, “We have no doubt the brothers were not acting alone. The devices used to detonate the two bombs were highly sophisticated and not the kind of thing people learn from Google.

“They were too advanced. Someone gave the brothers the skills and it is now our job to find out just who they were. Agents think the sleeper cell has up to a dozen members and has been waiting several years for their day to come.”

The FBI are retracing the elder brother Tamerlan's movements during a six-month trip he made to the Russia Caucusus last year, including a visit to Chechnya, a senior FBI counter-terrorism official told The Mirror.

Tamerlan's Russian-language YouTube page contains numerous links to extremist Islamic sites, including one that features an hour-long speech by the radical cleric Feiz Mohammed, along with other videos labeled "Terrorists" and "Islam".