"We believe this to be a terrorist," said Ed Davis, the Boston police commissioner.
"We believe this to be a man here to kill people."
Meanwhile, a series of dramatic pictures were beamed around the globe – from an early-hours gunfight to the uncle of the two suspects, delivering an impassioned plea for Dzhokhar to give himself up.
"Dzhokhar, if you are alive, turn yourself in and ask for forgiveness from the victims, from the injured and from those who left," Ruslan Tsarni said from outside his home in Maryland.
The pair were ethnic Chechens who lived in Russia's volatile Dagestan republic more than a decade ago before moving to the US with their family.
Mr Tsarni said his nephews had struggled to settle themselves in the US and ended up "thereby just hating everyone".
Asked what he thought provoked the bombings, he said: "Being losers – hatred to those who were able to settle themselves.
"These are the only reasons I can imagine. Anything else, anything else to do with religion, with Islam, it's a fraud, it's a fake."
The father of the suspects described his fugitive son as an "angel" in an anguished interview in which he claimed they were set up.
Anzor Tsarnaev spoke by telephone from Dagestan after Tamerlan was killed in the shootout.
"My son is a true angel," the elder Tsarnaev said. He said his son was "an intelligent boy" who was studying medicine.
"We expected him to come here on holidays," he said. "They were set up, they were set up! I saw it on television. They killed my older son, Tamerlan."
Dramatic scenes unfolded as it emerged that America's worst terrorist attack since September 11, 2001 had been carried out by young brothers who had been brought up in America by their parents. They appeared to have hatched their plot in a flat in Cambridge, Massachusetts, the home of Harvard University.
Dzhokhar, a University of Massachusetts student, was last night being hunted across the region, with officers focusing on the suburb of Watertown, nine miles from the bombings.
Relatives suggested that he had been led into jihadism by Tamerlan, a boxer and devout Muslim, who had publicly denounced Americans for having no "values" or "control".
John Kerry, the US Secretary of State, promised that authorities would win a "direct confrontation with evil" by bringing justice to the Tsarnaevs.
"We're part of the way there, and the president intends to finish the job," Mr Kerry said.
Hundreds of investigators were going door-to-door in Watertown to find the 19-year-old suspect, who was feared to be wearing a suicide vest after a trigger was discovered on his brother's body.
Teams of officers from the FBI, city police and state police swarmed the town of 30,000 people, several of whom posted photos of snipers and troops perched on their garden sheds.
They were joined by SWAT teams, explosives experts and dog handlers as federal and state authorities sought to prevent any further violence following Dzhokhar's escape.
The brothers were tracked down hours after the FBI released images of them
With Dzhokhar still known publicly only as "white cap" and Tamerlan as "black cap", gunfire at 10.30pm at the Massachusetts Institute of Technology in Cambridge soon turned out to be connected.
The brothers shot dead university policeman Sean Collier (26) who had only joined the MIT police force in January 2012. They fled the scene, stealing a car and taking its driver hostage. Later, at a petrol station, they freed the driver, who promptly alerted authorities.
Pursued by police, the attackers began throwing bombs and shooting at the officers, wounding one. Police fired back and killed Tamerlan, who Dzhokhar drove over to escape.
US intelligence agencies were last night looking into whether the brothers had received training or assistance from overseas extremists, after it emerged that Tamerlan had left the US for somewhere via Russia last year.
Associates said Tamerlan had changed drastically on his return, after which he began endorsing jihadist videos on YouTube. He had also grown a full beard, which he shaved before Monday's attack.