A visitor places a candle among flowers at the Columbine Memorial at Clement Park in Littleton, Colo. (Jason Connolly/AFP/Getty Images)

Twenty years ago Saturday, two teenage gunmen claimed the lives of 13 victims and marked the beginning of an era in which America has repeatedly been forced to reckon with the threat of school shootings. Since the Columbine High School massacre, more than 226,000 students at 233 schools have been affected by school shootings, according to a Washington Post analysis.

On April 20, 1999, two Columbine seniors, Eric Harris and Dylan Klebold, carried out a planned attack at their high school using a combination of firearms and homemade explosives. They shot and killed 13 people — 12 students and a teacher — before turning their weapons on themselves.

A memorial was designated in 2007 to honor the victims: students Cassie Bernal, Steven Curnow, Corey DePooter, Kelly Fleming, Matthew Kechter, Daniel Mauser, Daniel Rohrbough, Rachel Scott, Isaiah Shoels, John Tomlin, Lauren Townsend and Kyle Velasquez, and teacher Dave Sanders. The outdoor space, in a park adjacent to Columbine High School, welcomes visitors to reflect on the community’s loss.

This past week, three days of commemorative events were planned in honor of the victims, culminating with a memorial ceremony Saturday at the memorial in Littleton, Colo., a Denver suburb. The events continued despite a threat against the school earlier in the week: On Tuesday, the community faced danger once again after an 18-year-old woman traveled from Florida to Colorado and purchased a pump-action shotgun at a shop near Columbine High School.

The woman, Sol Pais, had left a trail of disturbing messages online. A manhunt ensued and the Jefferson County Public Schools ordered a lockout, keeping students inside for safety. As the manhunt stretched into a second day, classes were canceled. Pais was found dead Wednesday of an apparent self-inflicted gunshot wound.

The incident heightened already tight security around the week’s events.

Bailey Rosiere was a second-grader when the high school came under siege; she was one of hundreds who attended a vigil Friday night at the Columbine Memorial. Despite two decades of distance from the event, she told a local ABC station that the memory would never leave her.

“It makes it not just an empty or upsetting or sad feeling; it’s more of a deep impact,” she said. “Because you can’t ever forget no matter how young you were.”

Also in the crowd was Sarah Boyd, who came to lay flowers with her husband as she has done every year.

“It can happen anywhere. No one is immune, unfortunately,” Boyd told the Denver Post. She had graduated from Columbine in 1996 and was nearby when the attack began. “I hope someday that people can look back and say these are the things that were made better because of such a terrible day.”

At Saturday’s ceremony at the memorial, speakers discussed how the tragedy inspired strength and change.

“Our hearts have huge holes in them, but our hearts are bigger than they were 20 years ago,” said Dawn Anna, mother of slain student Lauren Townsend, according to Reuters.

Patrick Ireland, who fell from the school’s library window into the arms of firefighters in one of the iconic images of the attack, talked about his physical and emotional recovery, Reuters reported. “You’re a victim only if you allow yourself to become one,” he said.

Artist Makoto Fujimura symbolized that idea with a broken 17th-century Japanese tea bowl repaired with gold, the Associated Press reported, and Pastor James Hoxworth, from a church in nearby Lakewood, Colo., “urged anyone who was still struggling because of the shooting to reach out for help.”

Visitors to the memorial on Saturday left single flowers, cards and seed packets of columbines, Colorado’s state flower, the Associated Press reported.

Thirteen doves were released at the end of the ceremony.

Sheriff’s deputies patrolled the area on foot and by bike on a warm day as Little League games went on at nearby fields.

Ahead of Saturday’s memorial, politicians and public figures offered condolences for the victims. Sen. Cory Gardner (R-Colo.) tweeted that “our state and our country will never forget” the attack.

Former president Bill Clinton, who was in office when the shooting occurred, called for lawmakers to “pass sensible gun legislation” in an op-ed for the Denver Post this week, and also to reverse the political polarization surrounding gun safety.

Other Democrats and gun-control activists expressed condolences to the Columbine victims and survivors. Gabrielle Giffords, the former Arizona congresswoman who survived a 2011 assassination attempt in which six of her constituents were killed, tweeted, “my heart is with Littleton.”

But it was the remembrances from the victims’ families, friends and acquaintances that resonated most powerfully on the anniversary and in the days leading up to it.

Sanders, who was a coach at Columbine at the time of the shooting, was fatally shot after he ran into the school’s cafeteria to warn students to flee, saving hundreds of lives.

“He was an ordinary, yet extraordinary, person all at the same time,” his daughter, Coni Sanders, told PBS News Hour on Wednesday. “He loved to influence students and young athletes and get people to help turn their lives around if they were struggling.”

“I just want everyone to take a part of Dave Sanders with them,” she added, “and just move forward with kindness.”

More Columbine anniversary coverage:

How President Obama politicized the use of ‘thoughts and prayers’ after mass shootings

It’s been 20 years since the Columbine shooting. His job is to stop the next attack.

Perspective: School shootings didn’t start at Columbine. Here’s why that disaster became a blueprint for other killers.

Bullies and black trench coats: The Columbine shooting’s most dangerous myths

‘Infatuated’ with the Columbine shooting, she flew to Colorado and bought a gun