"We can't tolerate this anymore."

That's what President Barack Obama told those attending a memorial service Sunday in Newtown, Connecticut, two days after a man shot his way into Sandy Hook Elementary School and killed 26 people -- 20 of them children no older than 7, who would never go on a date, drive a car, marry or have kids of their own.

Obama offered his condolences, saying, "All across this land of ours, we have wept with you." He praised the residents of Newtown for having pulled together and "loved one another" with a spirit all could emulate. And he asked whether more could be done to prevent more parents, sisters and brothers, like those in this quiet New England town, from suffering similar heartaches.

"Can we honestly say we're doing enough to keep our children -- all of them -- safe from harm?" Obama said, adding that "if we don't get that right, we don't get anything right."

"If we're honest with ourselves, the answer is no."

His call to action capped an emotional prayer service at Newtown High School, in which local leaders of several religions -- a Jewish rabbi singing a prayer, a Muslim man choking back tears, and several Christian leaders offering perspective -- attempted to comfort a shattered community. Nine hundred watched in the school's auditorium, including several children toting teddy bears, and another 1,300 saw the proceedings from a nearby overflow room.

The aim was to show those suffering in Newtown they were not alone. With the help of their neighbors, they could move past this "act of unfathomable violence and destruction," explained the Rev. Matt Crebbin, senior minister at Newtown Congregational Church.

"We needed this," Crebbin said of the service. "We needed to be together."

That sentiment was echoed by Obama, who said the nation stands with Newtown. Then he went further than that, saying that the country owes it to them -- and to the people of Tuscon, Arizona; Oak Creek, Wisconsin; Aurora, Colorado, communities that also have been sites of mass shootings in the last two years -- not just to remember the victims, but take steps to prevent more bloodshed in the future.

The president didn't specify what steps he favors, but he did promise to put the power of his office toward preventing more senseless grief -- saying, "We can't accept events like this as routine."

"These tragedies must end. And to end them, we must change."

Clearer picture of what happened, but not why

The first calls came into police around 9:30 a.m. Friday.

Adam Lanza used "an assault weapon" to "literally (shoot) an entrance into the building," Connecticut Gov. Dannel Malloy said Sunday. The nightmare got worse as he moved through Sandy Hook's halls.

Using a Bushmaster AR-15 "assault-type rifle," the 20-year-old fired "multiple magazines" -- each of which contained 30 bullets -- to gun down six adults and children in two classrooms, said Connecticut State Police Lt. J. Paul Vance. He then took out a handgun and shot himself in a classroom as law enforcement officers approached, officials said.

All the victims were shot multiple times, said H. Wayne Carver II, Connecticut's chief medical examiner. Their deaths -- as well as that of Nancy Lanza, Adam's mother who suffered "multiple gunshot wounds" at their Newtown home -- are classified as homicides.

"This probably is the worst I have seen or the worst that I know of any of my colleagues having seen," said Carver, who did autopsies on seven victims.

Why did Lanza do it? That much, at least publicly, remains a mystery.

He had no criminal record. He and his mother, who collected guns, had visited a gun range at least once, ATF spokeswoman Ginger Colbrun said.

The few who spoke of Lanza publicly, including an aunt and former classmate, described him as very intelligent and quiet.