The brothers suspected of carrying out the deadly Boston Marathon explosions may have been planning more attacks, police say.
Tamerlan Tsarnaev was killed and Dzhokhar Tsarnaev is in a serious condition in hospital after separate gun battles with police.
But Boston police commissioner Ed Davis said officers cannot be certain there were not more explosives in the city.
"It's possible, but we have already searched the locations that are directly connected with the two bombers. They clearly had other explosives," Commissioner Davis said on Fox News Sunday.
"We feel they had plans to use those explosives," he added.
Meanwhile, the city's mayor Tom Menino has said authorities do not know if they will ever be able to question the surviving suspect.
Incendiary devices were detonated on the boat by police
Dzhokhar, 19, was captured late on Friday after a gunfight with officers that ended the city-wide search.
He was shot in the throat and could not speak because of injuries to his tongue, said a source close to the investigation, preventing police from questioning him.
The suspect is under armed guard in hospital, as investigators try to find a motive for the attack.
A video has been released by officers showing the operation to seize him after being tipped off by a man who noticed blood under a tarpaulin covering his boat in his garden.
Thermal imaging footage from a helicopter depicts a glowing white mass - Tsarnaev - hiding in the boat before he was detained by officers.
Bombing suspects Tamerlan Tsarnaev (L) and his brother Dzhokhar
One section of the video was of a police vehicle using an extended robotic arm to remove a tarpaulin covering the vessel while others showed officers detonating incendiary devices on the boat.
His brother Tamerlan, 26, died earlier on Friday after a shootout with the police. Officers are working to determine whether the two ethnic Chechen brothers accused of the attack acted alone.
A law enforcement source said Tamerlan travelled to Moscow in January 2012 and spent six months in the region.
But it remains unclear what he did while he was there, or whether he had contact with militant Islamist groups in southern Russia's restive Caucasus region.
A website used by North Caucasus rebels has reportedly denied any link to the marathon bombings.
Dzhokhar Tsarnaev leaves the boat he was hiding in
"The command of the Vilayat Dagestan mujahedeen... declares that the Caucasus fighters are not waging any military activities against the United States of America," the website said.
"We are only fighting Russia, which is not only responsible for the occupation of the Caucasus, but also for monstrous crimes against Muslims," the rebel site said.
Sky's Moscow correspondent Katie Stallard said the development was "very significant" because authorities will be looking for the ideology behind the attacks.
She said: "Until they are able to speak to Dzhokhar to get information from him, they will be working backwards through these men's history in terms of travel, where they have been physically, websites they have visited, material they have looked at online."
Police look around the corner as Dzhokhar was hiding in a boat nearby
Stallard added: "There are allegedly videos that the older brother Tamerlan had been looking at which identified him with the cause of the Caucases emirate.
"It's a separatist moment down in the south of Russia, started as a Chechen nationalist movement that has become a movement towards an independent Islamic state in that region."
The FBI, which interviewed Tamerlan in 2011 after he was flagged by Russian authorities, believes the older brother was the leader of the pair.
Investigators are checking on people who had contact with both brothers to see if anyone else was involved.
Dzhokhar had been hiding in the boat that was parked in the garden of a house in the suburb of Watertown. He was captured after the owner spotted blood on the vessel and called the police.
Massachusetts State Police used infra-red to spot Dzhokhar
Residents in Watertown have held a vigil to remember the three people killed and 176 others injured in the marathon attacks.
As night fell, those gathered at the Watertown High School lit candles as a mark of respect for the victims.
Investigators are trying to establish whether the brothers had assistance leading up to the detonation of bombs made in pressure cookers and packed with ball bearings and nails.
Dzhokhar is being treated at Beth Israel Deaconess Medical Centre in Boston.
It was unclear when he would be able to talk or when he would be charged.
"It's serious ... he's not yet able to speak," Massachusetts Governor Deval Patrick told reporters. "We have a million questions - and those questions need to be answered."
During their getaway attempt, the brothers apparently killed Massachusetts Institute of Technology policeman Sean Collier and severely wounded another officer.