An “armed and dangerous” suspect in a double killing and kidnapping has been captured, but the toddler he snatched is still missing, US police say.

The 19-year-old college student wanted over the Boston Marathon bombings is seriously ill in hospital and unable to be questioned after a manhunt that left the city virtually paralysed and his older brother and accomplice dead.

Boston Marathon bombing suspect Dzhokhar Tsarnaev has been captured alive and is in police custody, it has been confirmed.

A police officer shot on the campus at the Massachusetts Institute of Technology has died from injuries.