BOSTON (AP) - Investigators believe that two brothers suspected in the Boston Marathon bombing were likely planning other attacks based on the cache of weapons uncovered, the city's police commissioner said Sunday.

As Boston-area residents came together in prayer and reflection after a tumultuous week, the lone surviving suspect in the bombing lay hospitalised under heavy guard apparently in no shape for interrogation.

What 19-year-old Dzhokhar Tsarnaev will say and when are unclear. He remained in serious condition two days after being pulled bloody and wounded from a tarp-covered boat in a Watertown backyard. The capture came at the end of a tense Friday that began with his 26-year-old brother, Tamerlan, dying in a gun battle with police.

There was no immediate word on when Tsarnaev might be charged and what those charges would be. The twin bombings killed three people and wounded more than 180.

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.

Boston Police Commissioner Ed Davis told CBS' "Face the Nation" that authorities found an arsenal of homemade explosives after Friday's gun battle between police and the two suspects.

"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," Davis said. "That's my belief at this point." The scene of the gun battle was loaded with unexploded bombs, and authorities had to alert arriving officers to them and clear the scene, Davis said. One improvised explosive device was found in the Mercedes the brothers are accused of carjacking, he said.

"This was as dangerous as it gets in urban policing," Davis said.

US officials said the elite interrogation team would question Tsarnaev, a Massachusetts college student, without reading him his Miranda rights, which guarantees the right to remain silent and the right to an attorney.