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

Boston bombing suspect Dzhokhar Tsarnaev remained in serious condition in hospital on Saturday 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.

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

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

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.