Police release 'thermal image' pictures of Boston bomber

US police on Saturday released thermal image pictures of Dzhokhar Tsarnaev that they said helped to track the Boston bomber suspect before his capture.

Nineteen-year-old Tsarnaev was wounded and can be seen slumped on the bottom of a boat in which he is hiding from a mass manhunt late Friday.

The owner of the boat saw blood on the side of the boat and called police who surrounded the vessel which was on blocks in a back garden.

The bomb suspect surrendered two hours later but only after police had used a helicopter with the infra-red camera to check his movements and a final gunbattle.

Five pictures released by Massachusetts state police on their Twitter account give the most detailed images yet shown of Tsarnaev's state as he becomes aware that he is surrounded.

Thermal imagers detect heat so investigators can spot or track a body or other heat source inside a house or another surrounding through its walls. Police commonly use the cameras to see whether marijuana is being grown inside a house with heat lamps.

One of the police pictures shows Tsarnaev, who is now in serious condition in hospital, curled up on the floor of the boat. Others show the boat, including images of a robot lifting up the tarp cover so investigators could see inside.

The boat standoff started with a gunbattle between the suspect and his hunters.

According to police, Federal Bureau of Investigation (FBI) agents spent 20 minutes trying to negotiate with Tsarnaev before he gave up. "We know you're bleeding. We know you're tired," one negotiator is heard shouting in one tape played by local media.