Before the night was out, one of the young men was dead, crushed beneath his own hijacked getaway car, while the other cowered in a boat, bleeding heavily, as police closed in.
Tamerlan Tsarnaev, 26, and his brother Dzhokhar, 19, broke their cover hours after authorities released photographs of the suspects. It is unclear why they decided to remain in the area so long after Monday's attack.
The evening began to unravel when the brothers encountered Sean Collier, a 26-year-old police officer at Massachusetts Institute of Technology, according to accounts by police and government agencies.
Collier had been responding to a call of a disturbance at the university's Cambridge campus. Whether that call was connected to the brothers is unclear. Earlier reports that the pair had robbed a nearby convenience store were later withdrawn by the authorities.
In any case, Collier's body was found in his car at about 10.30 p.m. on Thursday. He had been shot multiple times, in what Boston Police Chief Ed Davis described as an assassination-style murder.
The brothers, meanwhile, were fleeing west across Cambridge to the nearby suburb of Watertown in a hijacked car. For a time, the car's owner was an unwilling passenger and listened as the pair told him that they had bombed the marathon earlier in the week.
After about half an hour, the brothers pulled into a gas station and forced the man to withdraw cash from an ATM before leaving him behind. Apparently unknown to them, police were tracking their movements using the man's cellphone, left behind in his car. Somewhere along the way, they stole a second a car.
AT LEAST SIX BOMBS
At about 12:30 a.m. on Friday, a police officer from the suburb of Watertown found the brothers, each now in his own stolen car, on a quiet street. Almost immediately, the brothers emerged from their vehicles and began firing, Edward Deveau, the chief of Watertown police, said in an interview with CNN on Saturday.
He said other officers arrived to find themselves in the middle of a gun battle, including a transit police officer who would be shot in the groin. About 200 rounds were shot in five or 10 minutes.
The brothers, armed with handguns and a rifle, also lobbed explosive devices, some resembling crude grenades, according to Deveau.
Police believe they had at least six bombs, three of which exploded, Deveau said. One was a pressure cooker bomb similar to a device used in the marathon bombing, Boston police said.
Towards the end of the battle, Tamerlan began walking towards the officers, shooting as he approached. Then, a few feet from the officers, his ammunition ran out, Deveau said.
He was tackled by the officers, who attempted to handcuff him in the street. Meanwhile, Dzhokhar had gotten back into a car and raced towards the group.
"One of them yells, 'Look out!', and here comes the black SUV, the carjacked car, directly at them," Devaux said.
The officers were able to dive out of the way. Tamerlan was not. He was hit by his brother's car and dragged a short way down the street, Deveau said, leaving a streak of blood in the asphalt that was still visible on Saturday, according to residents.
Tamerlan would later be pronounced dead at a hospital, while the younger brother disappeared into the night, leaving the car abandoned and fleeing on foot.
As the manhunt dragged on through Friday, residents of the Boston area were urged to stay indoors as officers in combat gear went house to house in a cordoned-off zone of about 20 streets in Watertown.
Even as authorities were announcing that the "stay-indoors" request was being lifted at about 6 p.m., a call came in to the Watertown police that there appeared to be someone hiding in a boat stored in a backyard about half a mile from the earlier shootout.
The hiding place was just outside of the perimeter of the manhunt during the day, police said.
Officers stormed the property around 7 p.m., and once again a flurry of gunfire reverberated on the streets of Watertown. Police lobbed stun grenades in an attempt to immobilize whoever was in the boat.
But police did not immediately rush the boat once the initial gunfire subsided. They said they hoped to take the suspect alive and were concerned that Dzhokhar might be carrying additional explosives or that the boat's half-full gas tank might be ignited.
As the siege dragged on for more than an hour, a police robot moved in to lift a plastic sheeting covering the boat.
An FBI negotiator stared down at the boat from the second floor of the house, relying on a helicopter flying overhead with heat-tracking devices to confirm that someone was still moving beneath the tarpaulin.
It took the negotiator 15 or 20 minutes, but, eventually, a badly injured Dzhokhar emerged from beneath the tarpaulin, lifting his shirt as instructed to show he was unarmed.
Dzhokhar, who had lost a considerable amount of blood, was loaded into an ambulance and rushed under police escort to Boston's Beth Israel Deaconess Medical Center in serious condition.