The brothers suspected of carrying out the deadly Boston Marathon explosions may have been planning more attacks, police say.
Boston police commissioner Ed Davis said officers found a huge cache of weapons and explosives at the scene of a shootout with both brothers.
Tamerlan Tsarnaev, 26, was killed in that gun battle, but his younger sibling Dzhokhar Tsarnaev is in a critical but stable condition after another shootout with police later that day.
Police are waiting to question Dzhokhar, 19, but he remains unable to speak as he has a gunshot wound to the throat.
The city's mayor Tom Menino said authorities do not know if they will ever be able to question him.
But Commissioner Davis said there were fears there were more explosives around Boston.
"It's possible, but we have already searched the locations that are directly connected with the two bombers. They clearly had other explosives," he told Fox News.
"We feel they had plans to use those explosives."
Dzhokhar, 19, was captured late on Friday, ending a city-wide search. He 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.
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.
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.
"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."
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.