Massachusetts authorities arrested the teen after a resident found him in his boat in Watertown. The man saw blood on his boat in the backyard, and the suspect bleeding under a tarp.
Police converged on the scene and asked the teen to surrender.
"Come out on your own terms. We know you're in there. Come out with your hands up," officers said.
Police and the suspect exchanged gunfire.
He was found in a boat in the yard of a home close to where he and his older brother engaged in a shootout with police nearly 24 hours earlier.
"We've closed an important chapter in this tragedy," President Barack Obama said after the arrest.
2 BROTHERS, 2 ENDINGS
The brothers came from the Russian Caucasus region and moved to Kazakhstan at a young age before coming to the United States several years ago. The younger brother was 8 years old at the time, according to relatives. He came to the United States as a tourist with his family in the early 2000s and later asked for asylum. He became a naturalized U.S. citizen last year, according to a federal official.
He attended the University of Massachusetts-Dartmouth.
The older brother was not a citizen, but was lawfully in the United States as a green-card holder. He studied engineering at Bunker Hill Community College just outside Boston, but had taken the year off to train as a boxer, sources told CNN.
The family of Katherine Russell, his wife, issued a letter saying their "hearts are sickened by the knowledge of the horror he has inflicted."
FBI agents interviewed the older brother two years ago at the request of a foreign government, but found no connection with terror groups. An agency official declined to to name the foreign government, but said the FBI took various investigative steps, including looking at his travel history and checking databases for derogatory information.