Authorities wondered: Were the two linked?
The carjacking victim had left his cell phone in the SUV, and police were using it to monitor the vehicle's every movement. It was now shortly after 12:30 a.m. Friday.
Many Watertown police officers had ended their shift at midnight and were headed home when they heard a possible suspect in the MIT shooting was in their vicinity.
"All they knew at the time was this was related to the MIT murder over there," Watertown Police Chief Edward Deveau told CNN.
Officer Joe Reynolds was the first to spot the stolen vehicle, driving in tandem with a Honda Civic. He notified the station that he had the suspects in sight.
"OK, do not try to pull their vehicle over until we get you some more backup," he was told.
Officer Reynolds continued to follow them. At least seven officers soon arrived.
On Laurel Street, a side road in the middle of a neighborhood, the two brothers stopped their vehicles and immediately started shooting. "They took the gunfight to us," Deveau recalled.
Officer Reynolds kicked his cruiser in reverse to try to distance himself from the suspects.
Andrew Kitzenberg lives in a three-story home at 62 Laurel, the location where the gun battle began. He saw two men crouched behind the SUV, opening fire on police.
One officer radioed dispatch: "They have explosives and grenades." Loud pops could be heard in the background.
"Shots fired! Shots fired!" he said.
Kitzenberg ran upstairs to his third-floor bedroom and peeked out a window, capturing the chaotic scene on his phone. At 12:55 a.m., he tweeted, "Shoot out outside my room in Watertown. 62 Laurel st."
The suspects grabbed a backpack, apparently from the Honda, pulled out a pressure-cooker bomb and placed it on the ground. An explosion rocked the neighborhood, and the brothers kept unloading on cops.
"Shoot out with 5 minutes of gun fire and pressure cooker bomb," Kitzenberg tweeted at 12:57 a.m.
He followed quickly with another tweet at 1 a.m.: "PD claiming IED's on the street. Everyone stay off of laurel st."
"I actually saw them light the bomb and I saw a spark from it. As soon as I saw that spark I hit the ground," he recalled.
Up to that point, Kitzenberg believed he was looking at an intense gun battle related to the MIT killing. "When they started using explosives, then I knew it was something much more significant and pretty much knew who I was looking at," he told CNN.
At one point, one of the police officers shifted his cruiser into gear as a diversionary tactic, and it rolled toward the two shooters. They fired round after round, blowing out the Ford SUV's back window and side window.
"I hope the chief's not mad at me. The cruiser's a little bit damaged," the cop told his captain.
The officer took up a position on the side and fired at Tamerlan and Dzhokhar.
The brothers tossed out five pipe bombs; two or three exploded.
After the final blast, a shroud of smoke covered the street and an armed Tamerlan ran toward police, shooting as he approached.
"He starts closing in on one of my officers, and they're literally about 10 feet away from each other, exchanging gunfire," Chief Deveau said. "Then, he runs out of ammunition."
Neighbor Kitzenberg gave a similar account: "He was running down the street, still engaging in gunfire. ... As he got closer to the officers, he was taken down.
An officer tackled the suspect. Two others joined in and tried to handcuff him.
All of a sudden, the black SUV roared down the street. "Get outta the way," an officer screamed. Police leaped to the side at the last second.
The younger brother slammed into Tamerlan, dragging him about 20 yards. "Ran over his brother," said Deveau.
A typical gunfight lasts about a minute, Deveau said. The one with the Tsnarnaev brothers lasted about 10 minutes, with more than 200 rounds fired and three bomb blasts.
A transit officer was badly wounded in the exchange. The Watertown police on scene immediately turned their attention to trying to save his life.
The black SUV with Dzhokhar behind the wheel managed to escape, with cops giving chase.
It was shortly past 1 in the morning.
A boat leads to the big break
Blood was found inside the Mercedes ditched on Spruce Street. More blood was discovered outside the SUV, an indication Dzhokhar was wounded.
That morning, authorities took the unprecedented measure of telling residents of greater Boston to stay indoors. Schools were closed. Restaurants shut for the day. One of the nation's largest cities became a ghost town, its streets clear of people and traffic.
In Watertown, the Massachusetts State Police, FBI, and local police conducted a door-to-door search of homes on about 20 streets. They came up empty handed.
By 6 p.m., authorities said they believed the suspect was still in the region but they lifted the order to stay inside.
After being cooped up all day, David Henneberry wanted to check on his boat -- his favorite toy, his "baby" -- named Slip Away II. He had noticed from his house that its cover had come slightly off.
It had irked him for much of the day. The winds had been unusually strong that day. Henneberry figured the winds had loosened the cover.
He climbed three steps up a ladder and saw "a good amount of blood," Henneberry told CNN-affiliate WCVB.