One officer tackled the older brother after he ran out of bullets walking down the street -- as if in Hollywood showdown -- firing upon police about 10 feet away.
But as police handcuffed the older brother, the younger brother barreled toward them in the carjacked SUV. Officers leaped for their lives.
The younger brother then drove over his older brother, dragging him a short distance down the street, police said.
Authorities didn't have an immediate explanation as to why he ran over his big brother.
The firefight produced at least one casualty: A transit police officer, Richard Donahue, was wounded in the groin. As it turned out, Donahue and Collier, the slain officer, graduated from the police academy together.
The younger brother -- now the only surviving brother -- ditched the car and escaped into the dark streets at 1 a.m. Friday.
As news outlets reported the dramatic shootout the next morning, the manhunt reached its highest intensity.
A major American city was turned into a virtual ghost town: Bostonians, and especially Watertown residents, stayed off the streets. This gave police an open field to notice anything out of the ordinary.
Meanwhile, the nation wondered, where was the Dzhokar Tsarnaev?
His discovery would turn on the curiosity of a citizen -- a homeowner -- who noticed something amiss late Friday with a tarp covering his boat, stowed in his backyard since winter.
Someone had cut a retention strap.
A closer inspection showed blood on the tarp.
Boat owner David Henneberry thought maybe a varmint had crawled inside, his stepson said.
But a peek inside revealed a pool of blood and a silhouette in the darkness of a something, or someone, curled up.
Henneberry's immediate call to 911, in effect, began the final chapter of the manhunt.
About 2,000 law officers and their technology swarmed the scene.
Another firefight between the suspect and police erupted.
In the night air, a police helicopter used infrared cameras to see Tsarnaev's movements inside the boat.
"The technology is called 'flir,' forward looking infrared," said former FBI assistant director Tom Fuentes, also a CNN contributor. "The human body is warmer than the air around him, so it stands out."
Knowing the fugitive was alive and moving, police negotiated with Tsarnaev.
Bloodied and injured, he surrendered that night, his capture made possible by police, thermal-imaging technology and a citizen's wisdom to run to the phone.