After the two brothers engaged in a gun battle with police early Friday, authorities surveying the scene of the shootout found it was loaded with unexploded homemade bombs. They also found more than 250 rounds of ammunition.
Police Commissioner Ed Davis said the stockpile was "as dangerous as it gets in urban policing."
"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. That's my belief at this point." Davis told CBS's "Face the Nation."
On "Fox News Sunday," he said authorities cannot be positive there aren't more explosives that haven't been found. But the people of Boston are safe, he insisted.
The suspects are two ethnic Chechen brothers from southern Russia -- 19-year-old Dzhokhar Tsarnaev and his 26-year-old brother, Tamerlan. Their motive remained unclear.
The older brother was killed during a getaway attempt. The younger brother, Dzhokhar Tsarnaev, remained hospitalized in serious condition Sunday after his capture Friday from a tarp-covered boat in a suburban Boston backyard. Authorities would not comment on whether he had been questioned, but several officials have said Tsarnaev's injuries left him unable to communicate, at least for now.
Shots were fired from the boat, but investigators haven't determined where the gunfire was aimed, Davis said.
The federal Bureau of Alcohol, Tobacco, Firearms and Explosives is tracing the weapons to try to determine how they were obtained by the suspects.
Tsarnaev could be charged as early as Sunday, although it was not clear 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.
Across the rattled city, churches opened their doors to remember the dead and ease the grief of the living.
At the Cathedral of the Holy Cross in South Boston, photographs of the three people killed in the attack and an MIT police officer slain Thursday were displayed on the altar, the faces illuminated by glowing white pillar candles, one for each person lost.
"I hope we can all heal and move forward," said Kelly McKernan, who was crying as she left the service. "And obviously, the Mass today was a first step for us in that direction."
A six-block swath of Boylston Street, where the bombs were detonated, remained closed Sunday, though police at the scene told pedestrians it was expected to reopen before Monday morning.
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. The FBI allowed church officials a half-hour Saturday to go inside to gather the priests' robes, the wine and bread for Sunday's service.
Trinity's Rev. Samuel T. Lloyd III offered a prayer for those who were slain "and for those who must rebuild their lives without the legs that they ran and walked on last week."
"So where is God when the terrorists do their work?" Lloyd asked. "God is there, holding us and sustaining us. God is in the pain the victims are suffering, and the healing that will go on. God is with us as we try still to build a just world, a world where there will not be terrorists doing their terrible damage."
Near the crime scene, Dan and Keri Arone were pushing their 11-week-old daughter, Alexandria in a stroller when they stopped along Newbury Street, a block from the bombing site, to watch investigators in white jumpsuits scour the pavement. Wearing his bright blue marathon jacket, Dan Arone said he had crossed the finish line 40 minutes before the explosions.
The Waltham, Mass., couple visited the area to leave behind pairs of their running shoes among the bouquets of flowers, hand-written signs and other gifts at a makeshift memorial on Boylston Street, near the police barriers.
"I thought maybe we'd somehow get some closure," Dan Arone said of leaving behind the sneakers. "But I don't feel any closure yet."
At Mount Auburn Hospital in Cambridge, surgeons said the Boston transit police officer wounded in a shootout with the suspects had lost nearly all his blood, and his heart had stopped from a single gunshot wound that severed three major blood vessels in his right thigh.
Richard Donohue, 33, was in critical but stable condition. He is sedated and on a breathing machine but opened his eyes, moved his hands and feet and squeezed his wife's hand Sunday.
In New York, thousands of runners donned "I Run for Boston" bibs during a 4-mile run in Central Park, one of a number of races held around the world in support of the victims of the marathon bombings.
Thousands of runners in the London marathon offered their own tributes to Boston's dead and wounded. The race began after a moment of silence for the victims, and many competitors wore black armbands as a sign of solidarity. Two runners finished carrying a banner that read "For Boston."
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