An investigation involving more than 1,000 federal, state and local law enforcement personnel quickly was launched. A breakthrough in the case came less than two days later, when FBI analysts, who had pored through thousands of videos and photographs taken from security cameras in the area where the attack occurred, pinpointed two male suspects. The FBI released surveillance-camera images of the men, whose identities were then unknown, on the evening of April 18.
That night at around 10:30, Sean Collier, a 27-year-old police officer at the Massachusetts Institute of Technology, was shot dead in his patrol car on the school’s Cambridge campus. Authorities later would link the murder to the Tsarnaev brothers, who allegedly attempted to steal the officer’s service weapon. Soon after Collier was killed, Tamerlan Tsarnaev allegedly carjacked a Mercedes SUV at gunpoint, taking the driver hostage and telling him he was one of the Boston Marathon bombers. Dzhokhar Tsarnaev followed behind in a Honda Civic before joining his older brother and the hostage in the SUV. The brothers drove around the Boston area with their hostage, forcing him to withdraw money from an ATM and discussing driving to New York City.
When they stopped at a Cambridge gas station, the hostage escaped and called police, informing them the SUV could be tracked by his cellphone, which was still in the vehicle. Shortly after midnight, police in the Boston suburb of Watertown spotted the suspects in the stolen SUV and Honda Civic and tried to apprehend them. A gun battle broke out on a Watertown street, with the Tsarnaevs exchanging fire with the police and hurling explosive devices at them. One officer was seriously injured by gunshots but survived. After Tamerlan Tsarnaev was tackled by police, Dzhokhar Tsarnaev drove the stolen SUV straight at them, running over his brother before speeding away. He abandoned the SUV nearby then fled on foot. A gravely wounded Tamerlan Tsarnaev, whose body was riddled with bullets, was taken to a hospital, where doctors were unable to resuscitate him.