While Cambridge and Watertown, two adjoining neighbourhoods north of the city centre, remained tense after a night of frenzied violence, Boston itself was a ghost town for most of the day, though movement restrictions were eased late last night.
Residents woke up to uncertainty, with students being confined in Harvard libraries and entire families removed from their homes by police as a precautionary measure.
Authorities insisted the net was closing on the surviving suspect, Dzhokhar Tsarnaev, as the city of about 1m people was told to stay indoors. Soldiers and heavily armed police officers patrolled the streets in armoured cars, but as temperatures rose, people began to venture out.
The ethnic Chechen’s elder brother, Tamerlan, was earlier killed in a shootout with police after a night of violence.
The pair killed a Massachusetts Institute of Technology police officer just before midnight local time on Thursday, and hurled explosives at police in a car chase and gun battle that went on into the small hours of the morning.
Tens of thousands of police are searching for Dzhokhar, 19, whose uncle pleaded with him yesterday afternoon to come forward and seek forgiveness.
Ruslan Tsarni, who lives in Maryland and said he hadn’t seen his nephews since late 2005, gave an impassioned statement to gathered media, accusing Tsarnaev of bringing his ethnicity to shame.
“If you’re alive, turn yourself in and ask for forgiveness from the victims, from the injured… ask forgiveness from these people,” Mr Tsarni said.
“He put shame on our family. He put shame on the entire Chechen ethnicity.”
Confusion reigned in Boston, with police SWAT teams, sharpshooters and FBI agents scrambling to buildings as police helicopters buzzed overhead. Authorities also searched trains.
“We believe this man to be a terrorist,” said Boston Police Commissioner Ed Davis. “We believe this to be a man who’s come here to kill people.”
The bombings on Monday killed three and wounded more than 180, tearing off limbs in a spray of shrapnel and raising the spectre of another terrorist attack on US soil.
Investigators have shed no light on the motive for the bombing.
Meanwhile, the FBI removed a computer from the US home of a woman reported to be the suspect’s sister. Police said she was cooperating with the investigation and was “heartbroken, surprised and upset,” though she told reporters she was not sure the accusations against her brothers were true.
Police did not release the woman’s name but public records list Alina Tsarnaev as living at the address.
The woman, speaking earlier through a crack in the door, said she was sorry for the families that lost loved ones, “the same way I lost my loved one”.
“I’m hurt for everyone that’s been hurt,” she said.
“He was a great person,” the woman added of her dead brother. “I thought I knew him. I never would have expected that from him. He is a kind and loving man. The cops took his life away just the same way he took others’ lives away, if that’s even true. At the end of the day, no one knows the truth.”
She said she had not seen him in a long time. “I have no idea what got into them,” she said.
West New York Police Director Michael Indri confirmed that the woman speaking to the FBI was a sister of the bombing suspects and she was cooperating with the FBI.
Vicki Colon, who described herself as a friend of Alina Tsarnaev, said Alina feared her older brother because “he used to beat her”.