Tamerlan Tsarnaev ran out of ammunition during the shootout and was tackled by officers. That's when the younger Tsarnaev drove the Mercedes toward the officers and his brother.
"They dive out of the way, and he (the younger brother) drives over his brother and drags him a short distance down the street," Deveau said.
Tamerlan Tsarnaev was later pronounced dead at the hospital. He was wearing explosives and a triggering device when he died, a source briefed on the investigation told CNN.
The FBI, while executing a search warrant at a residence believed to have been affiliated with Tsarnaev, took three people into custody for questioning Friday. It was unclear Saturday what information the FBI was seeking and whether the three were released.
'Pool of blood'
The dramatic end to the manhunt came when a Watertown man, cooped up in his house all day because of a "shelter in place" order, finally stepped outside when the order was lifted.
David Henneberry soon noticed the tarp covering the boat in his backyard was flapping in the wind and a retention strap was cut. He also noticed a small amount of blood on the tarp.
"He basically stuck his head under the tarp, noticed a pool of blood," Henneberry's stepson Robert Duffy told CNN.
Henneberry called 911, Duffy said.
Authorities arrived and evacuated Duffy's stepfather. Using a bullhorn, they called out to the suspect: "Come out with your hands up."
The man refused.
"We used a robot to pull the tarp off the boat," David Procopio of the Massachusetts State Police said. "We were also watching him with a thermal imaging camera in our helicopter. He was weakened by blood loss -- injured last night, most likely."
A gunfight ensued, with more than 20 rounds fired.
Authorities eventually rushed the boat and took the teen into custody.
Cheers and mourning
As word of the arrest spread, hundreds of residents swarmed the streets of Watertown and Boston, surrounding police cars and cheering them on.
But the celebrations were tempered by the deaths of four people this week, all allegedly by the hands of the Tsarnaev brothers.
Three spectators were killed in the marathon bombings, and Collier, the MIT police officer, was fatally shot early Friday. At least 57 people remained hospitalized Saturday afternoon, including three in critical condition, according to a CNN count.
On Saturday night as Collier's body was driven from the morgue to a funeral home, Boston-area police officers and firefighters lined the route to pay respect to their slain colleague.
The family of 8-year-old Martin Richard, who was killed in the bombings, issued a statement thanking the authorities and members of the public who helped track down the two suspects.