Police guard the Watertown house where Dzhokhar Tsarnaev was found. Credit:AP It is unclear whether investigators knew the suspects' names when they released photographs and videos of them along the marathon course about 5pm on Thursday. Within minutes, the images circulated worldwide on the internet and television networks. Law enforcement officials had hoped that wide distribution would bring clues. Instead, it appears to have jarred the Tsarnaevs into action. After apparently spending three days watching the aftermath of the bombings from nearby Cambridge, the two left their apartment, heavily armed. Whether they intended to flee the area or provoke a confrontation is unclear. The following account was provided by law enforcement officials involved in the manhunt or in the bombing investigation.

Move to safety: Police evacuate residents in Watertown. Credit:Getty Images Just after 10.30pm on Thursday, the pair walked up to a parked police car at the Massachusetts Institute of Technology, where Sean Collier, 26, a campus officer, was nearing the end of his shift. A security camera would later show two men approaching the car and speaking to the officer. Suddenly, one of the men was seen pulling a gun and shooting Mr Collier repeatedly, including once in the head. Found: An FBI officer stands guard near the boat. Credit:Reuters Some officers concluded that the shooting was an effort to provoke a larger confrontation with police. ''They were looking to start something,'' one said.

Mr Collier was found in his car by other police and taken to a hospital, where he was pronounced dead. By then, the Tsarnaevs had moved on. A SWAT team scours the streets of Boston's Watertown. Credit:Getty Images At 10.39pm, just blocks from the cruiser in which Mr Collier lay bleeding, the pair held up the driver of a black Mercedes-Benz SUV. Hijacking it, they drove through Cambridge to nearby Watertown, a middle-class suburb of 30,000 where they searched for ATMs. ''They tell the driver they're the bombers,'' said a law enforcement official. The Tsarnaevs stopped at three ATMs and got $800 cash from one, using the SUV owner's bank card. Denied cash at other ATMs, the suspects dropped off the vehicle owner at a petrol station.

''The guy was very lucky that they let him go,'' police spokesman David Procopio said. Just after midnight, police spotted the Mercedes and gave chase. It tore through the nearly empty streets of Watertown with as many as a dozen police cars in pursuit. The officers had to dodge home-made bombs hurled from the speeding Mercedes. At 12.50pm, it stopped in a residential neighbourhood in Watertown. The brothers opened fire, starting a gun battle witnessed by neighbours. One of them, Andrew Kitzenberg, 29, said he saw two men engaged in ''constant gunfire'' with police. After more than 200 rounds were traded over several minutes, some officers were out of ammunition and charged the brothers' position with their police car. The vehicle was disabled by gunfire from the Mercedes. Mr Kitzenberg said he saw one of the shooters toss a metallic object - possibly a pressure-cooker bomb similar to the ones used in the marathon attack - in the direction of the police line. It rolled a few metres before detonating harmlessly.

Tamerlan Tsarnaev, now out of his car, tried to lob a pipe bomb at police, but it exploded in his hand. Police tackled and tried to subdue him while he was firing a pistol. Dzhokhar Tsarnaev, apparently intending to help his brother, tried to ram the officers with the Mercedes. Instead, the officers lunged out of the vehicle's path and he ran over his brother and dragged him along the street before speeding off with police in pursuit. Officers found the Mercedes abandoned and quickly sealed off neighbourhoods in Watertown as they began a street-by-street search. Tamerlan Tsarnaev was taken to a hospital where he was pronounced dead on arrival.

Loading Late on Friday afternoon, state and local authorities said they did not know Dzhokhar Tsarnaev's position. Just after 6 pm the streets of Watertown came alive with sirens. Police had cornered the suspect, hiding in a boat behind a suburban home. WASHINGTON POST