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Hundreds of survivors, friends and family gathered at sunset Tuesday to pay an emotional tribute to the 13 people slain at Columbine High (search) five years ago in the worst school shooting in U.S. history.

Participants bowed their heads as four F-16 fighter jets soared over the grassy amphitheater in Clement Park, a few hundred yards and just out of sight from the suburban school. They rose in unison to cheer Anne Marie Hochhalter (search), who was paralyzed from the waist down in the attack and delivered a message of hope from a wheelchair.

"We all will never forget what happened that day, but we can move forward and prove that we are strong," she said.

Hochhalter told an estimated 1,200 people her mother committed suicide six months after the massacre, but added that she and other survivors have gone on to college and careers.

Five years ago, on April 20, 1999, Columbine students Eric Harris (search) and Dylan Klebold (search) slaughtered 12 classmates and a teacher before committing suicide. There have been other anniversaries, but many said this year seemed different, and more difficult.

"Certain days are harder than others. This is one of them," said Joe Kechter, whose son, Matt, was among those killed. "We were told by counselors that the fifth would be harder. I don't know the reason but it is true."

Before the service began, he fondly recalled the time he spent with his son: "I don't have any regrets that way. I wish there were other things we could have done today."

The names of the dead were read aloud before the crowd and a bell was rung each time. Many in the crowd had tears in their eyes.

"The most lasting tribute we could make to the treasured 13 we remember here tonight is to make the world a better place than when they left it," Principal Frank DeAngelis said.

The mother of slain valedictorian Lauren Townsend read to the crowd from a collage of memories honoring the dead -- how 14-year-old Steven Curnow loved the thrill of airplane turbulence, how 16-year-old football player Matt Kechter loved to roll cookie dough with his mother, how 17-year-old Rachel Scott loved the feel of a newborn chick, how 47-year-old teacher Dave Sanders was happiest when he was with his wife and grandchild.

"It's so difficult to believe that it is five years since we last held our loved ones," Dawn Anna said. "Now let our hearts see what our eyes cannot."

For hours before the memorial, people came and went and somber groups left flowers near the school.

Debbie Oetter, 48, wept as she climbed down a small hill nearby, clutching a Bible to her chest.

"I don't know why this year hit me so hard," said Oetter, whose daughter was dating John Tomlin the day he was slain. She planned to visit a Habitat for Humanity house built in Tomlin's name and a store that donated clothes to his family.

"This is my way of getting through today," she said. "It's been good to look at all the good things that have come out of the evil that day."

The school itself was empty Tuesday, its 1,700 students given the day off. The building has been overhauled since the tragedy, with a new library replacing the room where 10 of the students were slain. The old library was scooped out and replaced with an open atrium.

"If it could happen here, it could happen any place," said Steve Cowles of Colorado Springs, whose daughter and son survived the massacre. "These people should never be forgotten."

Meanwhile, administrators at a Sioux City, Iowa, high school gave parents the option of keeping students home Tuesday because of fears of a similar attack; about 600 did not attend.

A 15-year-old boy was arrested Monday for falsely reporting a catastrophe, a misdemeanor. The boy a year ago had worn a shirt to the school bearing the images of the teens responsible for the Columbine killings.

"The boy does not have a weapon. He does not have a plan," West High Principal Michael McTaggart said. "The boy verbally said an inappropriate thing at the wrong time and we're going to take appropriate action. You can't scream fire in a movie theater."