The black Mercedes SUV sped down Spruce Street going about 70 mph, the driver struggling to maintain control. The vehicle had a busted headlight and flat tire.
With each rotation of its tire, the SUV made a loud thud -- whop, whop, whop. Gunshots and sirens from police cars in pursuit added to the soundtrack echoing across the neighborhood.
Residents of this sleepy Boston suburb were rattled awake.
Max Kerman, once a football and basketball star at Watertown high, rushed outside from his second-floor bedroom. For sure, he thought, cars parked along the narrow street would get smashed.
Gunshots continued to ring out. It appeared the driver of the SUV was shooting at cops. Kerman hit the ground and dialed 911. He didn't get an answer, so he dialed again.
He was told to stay put. Officers in cars slowed at the top of the street where the road hooks.
"Keep going!" Kerman shouted. "Keep going!"
It was the early hours of Friday, April 19.
Just up the road, the driver ditched the stolen car and disappeared into the dark of night.
One of the largest manhunts in the nation's history was hurtling toward a conclusion after paralyzing a city and captivating the nation. Two brothers, Dzhokhar Tsarnaev, 19, and Tamerlan Tsarnaev, 26, would become known as the suspects in the Boston Marathon bombings that killed three people and wounded 264 others.
Plenty of questions remain about what might have motivated them. Where were they and what were they doing in the weeks and months before the bombings? Answers to those questions are still emerging. What is clear is the 24 hours that ended in one brother's death and the other's capture.
Details reveal a remarkable collaboration between law enforcement and the public.
A cop killing and carjacking
The city of Cambridge was best known for its multiculturalism and astute intellectuals who attended Harvard and the Massachusetts Institute of Technology. It was not your typical terror breeding ground.
Officer Sean Collier sat alone in his patrol car last Thursday around 10:30 p.m. The New York Times reported that an ambulance staffed with students had rolled by shortly before. He flashed his blue lights to say hi; they responded with red lights, according to the Times.
The 27-year-old officer was wearing a protective vest. Five hours earlier, the FBI had released photographs and surveillance video of the two marathon bombing suspects, hoping the public could identify them. Authorities were told to be extra vigilant.
Collier didn't have time to react when two men approached from behind, shooting. He was struck four to five times.
It took 13 minutes to reach the downed officer after 911 calls reported shots fired. Enough time for the suspects to get away.
"That is confirmed, a gunshot wound. CPR in progress," one first responder radioed back.
Collier was rushed to Massachusetts General Hospital where he was pronounced dead, making him the fourth fatality in the carnage in Boston.
Across the Charles River, several blocks from the shooting scene, a man who authorities believe was Tamerlan Tsarnaev approached a black Mercedes ML 350 SUV in the Allston neighborhood and tapped on the passenger window. The driver rolled down the window and the suspect reached inside, opened the door and brandished his handgun.
"Did you hear about the Boston explosion?" he said, according to an affidavit. "I did that."
He pulled the magazine from his weapon and showed he was loaded. "I am serious," he said.
The driver told the Boston Globe it was the start of a terrifying 90-minute odyssey across metro Boston, first with the younger brother trailing in a second car, before stopping in a neighborhood in East Watertown. It was there, the driver said, the two men transferred heavy objects into the Mercedes.
"Luggage," the driver thought, according to the Globe.
The younger brother, Dzhokhar, hopped in the back seat. Tamerlan took over the driving.
"They asked me where I'm from. I told them I'm Chinese," the carjacking victim told CNN-affiliate WMUR. "I asked them if they were going to hurt me. They said they won't hurt me. I was thinking, 'I think they will kill me later.'"
The man with the gun demanded cash. The carjacking victim handed over $45. They wanted more money so he gave the his ATM card and the passcode. Dzhokhar was seen on surveillance video at a Bank of America ATM at 11:18 p.m.
Both brothers were said to love high-end cars, and now they were in a carjacked Mercedes darting from Allston to Watertown to Cambridge. Eventually, the fuel tank neared empty.
The brothers mentioned something about New York as they drove, according to the driver. Authorities have since said the two planned to attack Times Square.
The Mercedes pulled into a Shell gas station on Memorial Drive. At that time of night, the station only accepted cash to pump gas. Dzhokhar went inside to pay. According to the Boston Globe, Tamerlan put his gun in the door pocket to fiddle with a navigation system.
The carjacking victim saw this as his opportunity to flee. "I jumped out of the car and ran away across the street," he told WMUR.
Tamerlan tried to grab him, "but I ran very fast." He could hear the man swearing as he sprinted for his life.
"It was very scary at that moment," he recalled. "For me, I'm so lucky."
He burst through the door of a nearby Mobil gas station so hard, the clerk was angry -- until he saw the panic on the man's face and heard his pleas to call police. The carjacking victim curled up in a back room and hid.
The men in the stolen SUV sped off.
Somewhere along the way, the two stopped to pick up their other car: a green Honda Civic with a Massachusetts tag 116GC7. Earlier in the day, authorities had put out an all-points bulletin for that exact vehicle.
A dramatic shootout in Watertown
"Heading to Watertown," the voice said over police dispatch.
Two hours had passed since the shooting at MIT. The next hour was followed by a bizarre carjacking.