Two-thirds of the students at Columbine High School (search) stayed home Friday because of threats penciled on the wall of two bathrooms and a sidewalk.

The graffiti warned that "harm would come to others" on Friday. The first threat came late last week. On Monday, school officials notified students and parents about the threat, said Rick Kaufman, a spokesman for the Jefferson County School District (search), which includes Columbine.

Sheriff's spokeswoman Jacki Tallman said the threats did not specify an individual or group. "They were very vague and very short," she said.

School officials locked down the high school on Friday, meaning students were not allowed to leave the campus and additional police were stationed inside buildings. No violence was reported.

But most students still chose to stay home. About a third of the district's 1,800 students attended, Kaufman said, compared to an average attendance of more than 90 percent.

Kay Leiner's oldest daughter, a junior at Columbine, stayed home. Her other daughter decided to attend class.

"We let them decide. Probably nothing will happen, but there's always that little part that says, 'You know it could happen,' so why even take the risk," Leiner said.

Columbine has received numerous threats via the Internet and telephone since teen gunmen Eric Harris (search) and Dylan Klebold killed 12 classmates and a teacher before committing suicide on April 20, 1999.

Leiner said the latest threats brought heightened fear because the source was unknown and the messages appeared inside the school, rather than on the Internet.

Some Columbine students said they were more upset with the vandals for creating a nuisance on campus.