LITTLETON, Colo. (AP) A Colorado community will mark the 20th anniversary of the attack on Columbine High School on Saturday with community service projects and a ceremony remembering the 13 people killed by teenage gunmen.

Saturdays events in and around the suburban community surrounding Columbine end a three-day slate of somber ceremonies honoring the 12 students and a teacher who were killed and lending support to their families, survivors of the attack and the schools students and staff.

The days surrounding the anniversary remain emotionally fraught for survivors of the attack, including hundreds who escaped the building without physical wounds. Some describe their response to the month as an April fog, dominated by their own memories of the sunny Tuesday in April that shocked the world.

Since 1999, American schools have tried to prevent a threat that had once been unthinkable.

Districts across the country formed teams to assess threats and cooperate with law enforcement on a response. Drills training students to evacuate their school or lockdown and hide from a shooter are routine. School security has become a multibillion-dollar industry, adding specialized doors, surveillance video and other technology.

This week brought a new demonstration of that burden as federal authorities led a manhunt for a Florida teen described as infatuated with the 1999 shooting who traveled to Denver on Monday and purchased a shotgun.

On Tuesday, authorities published the young womans name and photo after learning of her obsession with Columbine and the gun purchase. They said she had not made specific threats but dozens of schools, including Columbine, locked their doors Tuesday.

More than 400,000 kids stayed home on Wednesday when schools shut down across the metro area.

The 18-year-old was discovered dead of an apparent suicide Wednesday morning in the foothills west of Denver, about 40 miles (64 kilometers) from Columbine.

Long-planned events marking the anniversary continued as scheduled, beginning with a Thursday evening church service and a community vigil Friday night at a memorial constructed within sight of the school.

The Columbine perpetrators, who took their own lives during the attack, have inspired cult-like admirers including some who have committed other shootings or were prevented from doing so.

Officials overseeing security at Columbine and other schools in Jefferson County acknowledged the dark interest this week and warned off those who would treat the school as a destination.

We are not a place to come visit if youre not a student, if you dont have business there, John McDonald, security chief for the school district, said Wednesday. Were not a tourist attraction and were not a place for you to come and gain inspiration.

Security remained heightened at Denver-area schools through the week. People who plan to attend the public remembrance ceremony Saturday afternoon at a park near Columbine also have been warned of security checkpoints.

The school itself will be closed to the public.