"What choice do we have?" he said. "Are we really prepared to say that we're powerless in the face of such carnage, that the politics are too hard?
"Are we prepared to say that such violence visited on our children year after year after year is somehow the price of our freedom?"
For Obama, that was an unmistakable sign that he would at least attempt to take on the tough issue of gun control. He made clear that the deaths compelled the US to act, and that he was the leader of a nation that was failing to keep its children safe.
He spoke of a broader effort, never outlining exactly what he would push for, but expressed outrage by yet another shooting rampage.
"Surely we can do better than this," he said. "We have an obligation to try."
In a vigil for the fallen, 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 elicited horror around the world, soul-searching in America, and fresh political debate about gun control. Privately, Mr Obama told Connecticut governor Dannel Malloy that Friday was the most difficult day of his presidency.
"Can we 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," the president said, sombre and steady in his voice.
"And if we're honest with ourselves, the answer is no. We're not doing enough and we will have to change."
He promised in the coming weeks to talk with law enforcement, mental health professionals, parents and educators in an effort to prevent mass shootings. The killings have restarted a debate in Washington about what politicians can to do help -- gun control or otherwise. Obama has called for "meaningful action" to prevent killings.
Democratic lawmakers said yesterday that military-style assault weapons should be banned and that a national commission should be established to examine mass shootings in the US.
Obama and Senate Democrats haven't pushed for new gun controls since rising to power in 2008. Outspoken advocates for stricter laws, including Senator Dianne Feinstein, say that's because of the powerful sway of the National Rifle Association, the gun owners' lobby.
But gun control advocates also say the latest shooting is a tipping point that could change the dynamic of the debate. Feinstein said she will propose legislation next year that would ban big clips, drums and strips of more than 10 bullets.
Near the start of his remarks, Obama read the names of the adults who died. He finished by reading the first names of the children.
"That's when it hit home," said Jose Sabillon, who attended the memorial with his son, Nick, a fourth-grader who survived the shooting unharmed.
Obama said of the children: "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.
"We're halfway between grief and hope," said Curt Brantl, whose daughter was in the library of the elementary school when the shootings occurred. She was not harmed.
The president first met privately with families of the victims and emergency personnel.
The gathering happened at Newtown High School, the site of Sunday night's interfaith vigil, about a mile and a half from where the shootings took place. Police and firefighters got standing ovations when they entered.
The Rev Matt Crebbin, senior minister of the Newtown Congregational Church. "We needed to be together to show that we are together and united."