(BOSTON) -- The final violent hours of the Boston terror spree began when the Tsarnaev brothers realized they were being followed by a Watertown police officer and stopped their car to attack the officer with gun fire and to heave a pressure cooker bomb at him.
The manhunt ended nearly 20 hours later when the surviving brother, badly wounded in the throat, meekly pulled up his shirt to show officers who had cornered him that he was not wearing a suicide vest.
The gripping chase of Tamerlan Tsarnaev, 26, and Dzhokhar Tsarnaev, 19, was described to ABC News on Sunday by Watertown Police Chief Edward Deveau.
The chief told ABC News that when the younger Tsarneav was finally in custody there was a great relief because police were certain that the duo intended to kill more people.
"These guys did the Boston Marathon bombing, they did not flee," Deveau said. "They had their pictures put into the media all over the world. They did not flee. They still had bombs...They had a plan to kill more people."
"That was clear from what they had on them. We have the pressure cooker bomb and we have five other what we were referring to as hand grenades -- what they can lob over -- what they made themselves. Six explosive devices that we recovered so far that evening... So their plan was to kill more people," the chief said.
The final hours began Thursday night after the brothers allegedly ambushed and killed a Massachusetts Institute of Technology police officer and then carjacked a vehicle.
"They told him that in no uncertain terms that they were the Boston Marathon bombers, that they had just killed a police officer and they would kill him if he didn't give them his car," Deveau said.
For a while, the older brother drove the carjacked vehicle and the younger brother drove his own Honda, Deveau said. They forced the car owner to withdraw at least $800 from an ATM, but eventually the car owner was able to escape and police used his cellphone to track the car, according to Deveau.
When a Watertown cop spotted the duo, his supervisors told him to not confront them and to not turn on his lights or siren until backup arrived.
"Unfortunately, they brought their fight to us. They stopped their car because they knew he was behind them. And immediately started shooting at my police officer," Deveau said.
"We're still trying to determine all the firearms that were down there. My understanding is that there were firearms and there was a long rifle," Deveau said.
"Our other officers were responding immediately. They were seconds behind him. So immediately we had three or four Watertown police officers in a gun fight with these two brothers," Deveau said
"At some point they go behind the Honda, open the trunk and start heaving a device at our officers. And there was a huge explosion. We believe that was the pressure cooker bomb that went off. We found the pressure cooker lid embedded into a car further down the street," he said.
Tamerlan Tsarnaev was fatally wounded in the shoot out, but Dzhokhar Tsarnaev slipped away. Blood in his car and blood behind a house where he apparently hid for a bit indicate that he was also wounded in the gunfight with police.
Bloodhounds were brought in to track the fugitive, but Deveau said so many people had been in the area that the dogs were unable to pick up Dzhokhar's scent.
Police didn't find him until about 8 p.m. Friday when David Henneberry noticed blood on his backyard boat and peaked in to see a bloody Dzhokhar.
Police quickly surrounded the boat and there was a brief but ferocious volley of fire.
"The boat's not going to float for awhile," Deveau said.
"It was a long period. There was probably a 10-15-minute period where they are trying to get [Dzhokhar] to pay attention. So negotiations went on. There was no conversation back and forth. At the end they were just making demands of him: Show your hands, lift your shirt. And eventually that's what he did," Deveau said.
"He was very slow and lethargic in every move that he made and they could see that there was no device on his chest. They kept creeping closer to him and then they felt it safe enough to pull him away from the boat," he said.
"The concern was what was in that boat. Was there more explosive devices? Was there more firearms in that boat? So, we needed to get him away from that to make it safe," Deveau added.
Deveau believes the wound to Dzhokhar's throat occurred while he was in the boat, but he can't be certain at this point.
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