"What choice do we have?" the president said. "Are we really prepared to say that we're powerless in the face of such carnage, that the politics are too hard?"
In a vigil for the fallen, in a moment of grief that spread around the world, he conceded that none of his words would match the sorrow, but he declared to the Connecticut community, site of the second-deadliest school shooting in US history: "You are not alone."
For Mr Obama, ending his fourth year in office, it was another sorrowful visit to a community in disbelief.
The massacre of 20 children and six adults at Sandy Hook Elementary School on Friday elicited horror around the world, soul-searching in the United States, fresh political debate about gun control, and questions about the incomprehensible - what drove the suspect to act.
Privately, Mr Obama told Connecticut governor Dannel Malloy that Friday was the most difficult day of his presidency.
Authorities said yesterday that the gunman in the shooting rampage was carrying an arsenal of hundreds of rounds of especially deadly ammunition - enough to kill just about every student in the school if given enough time, raising the chilling possibility that the bloodbath could have been far worse.
Adam Lanza shot himself in the head as he heard police drawing near to the classroom where he was slaughtering children, but he had more ammunition at the ready in the form of multiple, high-capacity clips each capable of holding 30 bullets.
The chief medical examiner said the ammunition was the type designed to break up inside a victim's body and inflict the maximum amount of damage, tearing apart bone and tissue.
Newtown officials could not say whether Sandy Hook Elementary School would reopen. The school district is considering sending surviving pupils to an empty school in nearby Monroe, but for many parents it was much too soon to contemplate resuming school day routines.
Jim Agostine, superintendent of schools in nearby Monroe, said plans are being made for children from Sandy Hook to attend classes in his town this week.
A Connecticut official said yesterday that the gunman's 52-year-old mother Nancy was found dead in her pyjamas in bed at the home they shared, shot four times in the head with a .22-calibre rifle.
The killer then went to the school on Friday morning with guns he took from his mother, got inside by breaking a window and began blasting his way through the building.
All the victims at the school were shot with the rifle, at least some of them up close, and all were apparently shot more than once, chief medical examiner H Wayne Carver said. There were as many as 11 shots on the bodies he examined. Lanza died of a gunshot wound to the head from a 10mm gun, said the official who described the scene at the mother's house.
Amid the confusion and sorrow, stories of heroism emerged, including an account of the principal Dawn Hochsprung, 47, and the school psychologist, Mary Sherlach, 56, rushing toward Lanza in an attempt to stop him. Both died.
There was also 27-year-old teacher Victoria Soto, whose name has been invoked as a portrait of selflessness. Investigators told relatives she was killed while shielding her first-graders from danger. She reportedly hid some students in a toilet or closet, ensuring they were safe, a cousin, Jim Wiltsie, told ABC News.
Federal agents have concluded that Lanza visited an area shooting range, but they do not know whether he actually practised shooting there.
Ginger Colburn, a spokeswoman for the federal Bureau of Alcohol, Tobacco and Firearms, would not identify the range or say how recently he was there.
Agents also determined that Lanza's mother visited shooting ranges several times, but it is not clear whether she took her son to the range or whether he ever fired a weapon there, Ms Colburn said.
Investigators have offered no motive for the shooting, and police have found no letters or diaries that could shed light on it.
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Mr Obama said at an evening vigil in Newtown that in the coming weeks, he would use "whatever power this office holds" to engage with law enforcement, mental health professionals, parents and educators in an effort to prevent more tragedies.
He told the audience: "Can say that we're truly doing enough to give all the children of this country the chance they deserve to live out their lives in happiness and with purpose? I've been reflecting on this the last few days. If we're honest without ourselves, the answer is no. And we will have to change."
He promised to lead a national effort, but left unclear what it would be, and how much it would address the explosive issue of gun control.
As Mr Obama read some of the names of victims early in his remarks, several people broke down, their sobs heard throughout the hall.
He closed his remarks by slowly reading the first names of each of the 26 victims.
"God has called them all home. For those of us who remain, let us find the strength to carry on and make our country worthy of their memory," he said.
The president had earlier privately met families of the victims and the emergency personnel who responded to the shootings. That meeting happened at Newtown High School, the site of last night's interfaith vigil, not far from where the shootings took place.
Police and firefighters got hugs and standing ovations when they entered, as did Mr Obama.
"We needed this," said the Rev Matt Crebbin, senior minister of the Newtown Congregational Church. "We need to be together here in this room. We needed to be together to show that we are together and united."