As the surviving brother of the two Boston Marathon bombing suspects remained seriously ill and unable to answer questions over, further details emerged about the older brother's alleged links to militant groups.
Questions are also being asked of how US law enforcement officials failed to follow up on leads on the threat posed by the ethnic Chechen brothers and the capacity of the American intelligence community to investigate potential risks.
Dzhokhar Tsarnaev (19) remained in a serious condition yesterday and under watch by armed guards after being found wounded and hiding in a covered boat in the back garden in Watertown near the city of Boston on Friday night less than 24 hours after his brother Tamerlan (26) died following a gun battle with police.
'May never speak'
Boston mayor Tom Menino said that Dzhokhar, who has lived in the Boston area for more than a decade, was in a "very serious" condition and may never be able to speak to investigators.
"We don't know if we'll ever be able to question the individual," Menino told ABC's This Week programme.
The suspect in last Monday’s bombings, which killed three people and injured more than 170, is too ill to be interrogated by investigators trying to determine a motive for the attacks, the extent of their plot and the possible involvement of others.
The teenager was being treated at Beth Israel Deaconess Memorial Hospital, where many of Monday's blast victims were taken, and was heavily sedated with a breathing tube in his throat. He is said to have been shot in the throat.
Investigators plan to question Dzhokhar, who is reported to have become resentful of the US and increasingly radicalised as a Muslim, about a six-month trip Tamerlan made to Dagestan in southern Russia last year.
US law enforcement officials confirmed that the FBI interviewed Tarmelan after a request from the Russian government in 2011 but did not find any domestic or foreign terrorist activity.
NBC News said yesterday that Russia’s internal security service contacted the FBI in November with questions about Tamerlan and handed over a copy of a case file on him, but the bureau never responded.
The 26-year-old Chechen was spotted meeting a person involved in a militant Islamic group in the Russian region of Dagestan. Tamerlan met the individual six times.
Boston city's police commissioner Ed Davis said the brothers were probably planning further attacks as they were carrying homemade bombs and grenades when police cornered them in Watertown in the early hours of Friday morning.
More than 250 rounds of used ammunition were found at the scene, while the ground was “littered with unexploded improvised explosive devices”, he said.
Massachusetts governor Deval Patrick said that surveillance video taken from the scene of Monday's marathon blasts showed Dzhokhar at the scene of the second explosion.
“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,” he said.
“It’s pretty clear about his involvement and pretty chilling, frankly.”