The news came as the surviving suspect was said to have indicated that he and his dead elder brother had acted alone, not in concert with an international terrorist group, and that they were motivated by US military action in Iraq and Afghanistan.
Three people died when two pressure-cooker bombs went off near the Marathon finish line last week. Initial estimates put the number of injured at around 170 to 180 but the Boston Public Health Commission said the figure had risen to 282.
The reason given was that dozens of victims with minor wounds who had delayed seeking treatment had begun to come forward as their injuries persisted. “One of the best examples is hearing issues,” a Commission spokesman, Nick Martin, told the Boston Globe.
Official sources quoted by US media said the surviving suspect, 19-year-old Dzhokhar Tsarnaev, who is facing the death penalty after being charged for using a weapon of mass destruction, had indicated to interrogators that he and his 26-year-old brother, Tamerlan, had acted alone.
Tamerlan was killed following a police shootout in the early hours of Friday morning, while Dzhokhar was captured later on Friday following a massive manhunt.
While he remains at Boston’s Beth Israel Deaconess Medical Centre, CNN reported that interrogators had conducted a preliminary interview in which he communicated by nodding and writing.
NBC reported that he and his brother learnt how to make bombs on the internet, while the Washington Post claimed US military action was their inspiration for the attack.
On Monday Dzhokhar was charged as he lay in his hospital bed. According to a transcript, he was able briefly to talk, despite fears that the injuries he sustained during the hunt would prevent him from doing so.
When asked, “Can you afford a lawyer?”, his response is recorded as: “No.”