The surviving fugitive was also feared to be strapped with explosives. Adding to the terror, an explosive was found in Boston Friday morning and disabled, an official said.
The suspects' uncle, Ruslan Tsarni, called his nephews "losers" and urged Dzhokhar, a naturalized American citizen of Chechen origin, to turn himself in.
"We're ashamed," he said adamantly.
An alert since two bombs tore through crowds at the Boston Marathon on Monday -- killing three people and wounding about 180 -- reached a feverish peak in the hours after the Federal Bureau of Investigation (FBI) released pictures of the two on Thursday.
The brothers, legal residents of the United States, were spotted in a convenience store near the Massachusetts Institute of Technology (MIT) in Cambridge late Thursday. They then went to the prestigious university where a campus police officer was killed in a first shootout, according to police.
A car driver was held up at gunpoint, but the driver was freed and they took the car to Watertown, which was sealed off by more than 9,000 police armed with shotguns, assault rifles and other weapons.