Boston's police commissioner has said the two suspects in the deadly marathon bombing had such a large cache of weapons that they were probably planning other attacks.
Ed Davis's revelation came as the surviving suspect, 19-year-old college student Dzhokhar Tsarnaev, remained unable on Sunday to speak with a bullet wound to the throat after his capture from a tarpaulin-covered boat in a suburban backyard.
The suspects in Monday's twin bombings at the Boston Marathon finishing line that killed three and wounded more than 180 are two ethnic Chechen brothers from southern Russia who had been in the US for about a decade - Dzhokhar Tsarnaev and his 26-year-old brother, Tamerlan.
Authorities found many unexploded homemade bombs at the scene of the brothers' gun battle early on Friday with police, along with more than 250 rounds of ammunition.
The stockpile was "as dangerous as it gets in urban policing", Davis said.
"We have reason to believe, based upon the evidence that was found at that scene - the explosions, the explosive ordnance that was unexploded and the firepower that they had - that they were going to attack other individuals," he told CBS.
Davis told Fox News that authorities could not be positive there were not more explosives somewhere that had not been found.
But he insisted the people of Boston are safe.
No motive offered
The ABC and NBC news networks reported late on Sunday that Dzhokhar Tsarnaev had regained consciousness and was responding in writing to questions put to him by authorities.
He was being watched by armed guards in the intensive care unit of Boston's Beth Israel Deaconess Medical Centre.
Al Jazeera's Alan Fisher, reporting from Boston, said earlier Dzhokhar Tsarnaev was in a stable condition.
We have reason to believe, based upon the evidence that was found at that scene - the explosions, the explosive ordnance that was unexploded and the firepower that they had - that they were going to attack other individuals.
"We know that Dzhokhar Tsarnaev lost a lot of blood," he said.
Investigators have not offered a motive for the marathon attack.
Tamerlan Tsarnaev died after a Thursday night firefight with police on the streets of Watertown, the Boston suburb where authorities finally cornered Dzhokhar Tsarnaev after a manhunt that shut down greater Boston on Friday.
Authorities would not comment on whether Dzhokhar Tsarnavev had been questioned.
It was not clear whether he was shot by police or wounded himself.
In the final standoff with police, shots were fired from the boat, but investigators have not determined where the gunfire was aimed, Davis said.
Tsarnaev could be charged any day.
The most serious charge available to federal prosecutors would be the use of a weapon of mass destruction to kill people, which carries a possible death sentence. Massachusetts does not have the death penalty.
US officials said an elite interrogation team would question Dzhokhar Tsarnaev without reading him his Miranda rights, which guarantees the right to remain silent and the right to an attorney.
Such an exception is allowed on a limited basis when the public may be in immediate danger, such as instances in which bombs are planted and ready to go off.
The federal public defender's office in Massachusetts said it had agreed to represent Dzhokhar Tsarnaev once he was charged.
Link to fighters denied
In an interview with the Associated Press news agency, the parents of Tamerlan Tsarnaev insisted on Sunday that he came to the southern Russian regions of Dagestan and Chechnya from January to July last year to visit relatives and had nothing to do with fighters operating in the region.
His father said his son slept much of the time.
The family emigrated to the US about a decade ago.
The brothers spent their early years in a small community of Chechens in the Central Asian country of Kyrgyzstan, a mainly Muslim nation of 5.5 million.
They moved in 2001 to Dagestan, a southern Russian province where their parents now live.
When the two suspects were identified, the FBI said it reviewed its records and found that in early 2011, a foreign government - which law enforcement officials confirmed was Russia - had asked for information about Tamerlan Tsarnaev.
The FBI said it was told that he was a "follower of radical Islam" and was preparing to travel to this foreign country to join unspecified underground groups.
The FBI said that it responded by interviewing Tamerlan Tsarnaev and family members but found no terrorism activity.
No evidence has emerged since to link Tamerlan Tsarnaev to armed groups in Russia's Caucasus.
Weapons under scrutiny
The Caucasus Emirate, which Russia and the US consider a terrorist organisation, on Sunday denied involvement in the Boston attack.
In interviews with officials and those who knew the Tsarnaevs, a picture has emerged of Tamerlan Tsarnaev as someone embittered towards the US, increasingly vehement in his Muslim faith and influential over his younger brother.
The federal Bureau of Alcohol, Tobacco, Firearms and Explosives was tracing the suspects' weapons to try to determine how they were obtained.
Neither of the brothers had permission to carry a gun.
Against this backdrop, Boston churches opened their doors on Sunday to remember the dead and ease the grief of the living.
Boston's historic Trinity Church could not host services Sunday because it was within the crime scene, but the congregation was invited to worship at the Temple Israel synagogue instead.