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.