At a memorial service Saturday to mark the anniversary of the massacre in Littleton, Colorado -- in which teenaged shooters Dylan Klebold and Eric Harris killed 12 Columbine High School students and a teacher before turning their guns on themselves -- there were reflections on grief, loss and healing.

"While time may lessen the intensity of the trauma, the incredible memories still remain," said former Columbine principal Ron Mitchell.

"The strength and resilience of this community has given us hope."

The events of that day planted a jarring and lasting sense of vulnerability in Americans' minds.

School shootings, once almost unheard of, have become a tragic feature of American life -- requiring all schools to tighten security, forcing even six-year-olds to take part in traumatic "live-shooter" drills.

Places that had always seemed the safest -- schools, libraries, even churches and synagogues -- no longer seem so.

A rededication 

But as folks in Littleton prepared for the official memorial service, there were also signs of a rededication to making things better.

Saturday was the third annual Columbine Day of Service, with hundreds of students and teachers donating time and labor to help charitable causes and honor the memory of those who died.

"Right after Columbine, people wanted to be better people," Dawn Anna, mother of 18-year-old victim Lauren Townsend, told The Denver Post.

"There was a real change in people's attitudes."

On Friday, survivors of the massacre joined friends and relatives of the victims at a prayer vigil in a memorial park, lighting candles and laying flowers on plaques bearing victims' names.

'I've just been angry'

One survivor, Amanda Duran, who was 15 at the time of the attack, said she could not believe more has not been done to control gun violence.

"You would have thought someone would have come with more gun laws" or required mental health evaluations of would-be gun buyers, she said Friday.

"But nothing of that sort happened. So I have just been angry and pissed off."

Her best friend was killed in the shooting.

School officials, student leaders and graduates, as well as Colorado Governor Jared Polis, spoke at the official memorial service in Littleton, in a park next to the high school. A recorded message from Bill Clinton, who was US president at the time of the massacre, was played.

Gun ownership grows

The shooting sparked a fierce national debate over gun rights and violence. Yet three presidents after Clinton, it seems no closer to resolution.

The number of firearms in circulation has continued to grow (now at 393 million in a nation of 326 million people), and mass shootings have become a disturbingly regular part of American life.

Since Columbine, an estimated 226,000 children in 233 schools have been exposed to the sight or sound of gunfire, according to a Washington Post investigation.

The worst shootings to date were those at Sandy Hook Elementary School in Newton, Connecticut in 2012 (20 young children and six adults were killed) and at Marjorie Stoneman Douglas High School in Parkland, Florida last year (17 dead).

But the power of an influential gun lobby and a long tradition of gun ownership have meant that little has been done to improve gun safety.

In the meantime, Columbine has taken on an almost mythical dimension in the memories of some.

A dark fascination with the killings by Klebold and Harris -- whose motives remain somewhat murky -- maintains a grip on some, as shown by the recent case of 18-year-old Sol Pais.

Police had been warned that the young Floridian, "fascinated" by Columbine, was headed to Colorado and had purchased a gun. Hundreds of schools in the Littleton area were protectively closed Tuesday.

But in the end, Pais was found in a remote, mountainous spot west of Denver -- having shot and killed herself.