Friends and family members of those killed in the Columbine (search) school shooting marked the six-year anniversary of the massacre Wednesday by visiting victims' grave sites and placing flowers on black granite crosses honoring the dead.

No formal observances were planned in the memory of the 12 students and one teacher who were gunned down in 1999 by students Eric Harris (search) and Dylan Klebold at the suburban Denver high school. The two teens then shot and killed themselves.

Friends and family members visited a cemetery memorial for the victims and brought flowers, pausing among the 5-foot-tall crosses and 13 memorial trees blossoming with white flowers. Two of the victims are buried there.

Cindy Thirouin, whose stepfather, Dave Sanders (search), died that day, brought red roses for each cross. She and her two children came to celebrate his life, not to mourn, she said. "After six years, it's getting easier, and it's time to move on," she said.

Columbine teachers gathered at the school Wednesday for a quiet remembrance. The campus was closed to outsiders and students were given the day off, although the building was open to victims' families. Teachers reported for an in-service day and had breakfast together.

Some teachers gathered in the library, where many of the victims died, for a moment of silence. Principal Frank DeAngelis read aloud the victims' names.

Of the roughly 140 teachers and staff who taught at Columbine at the time of the shootings, about 30 remain.

"It's difficult because it is a reminder," said Jefferson County schools spokesman Rick Kaufman of the anniversary. "Canceling classes is an opportunity for us to remember the victims — not necessarily the tragedy, but those 13 that died. It's a reflection back but we're also looking forward."