WELDON SPRING, Mo. (AP) -- Law enforcement officials on Tuesday were investigating a threatening message left in a school bathroom that referenced Columbine High School and kept about 900 high schoolers from attending class on Monday.

About half the student population of Francis Howell High School near Weldon Spring stayed home from school.

"The message left said something to the effect, don't come to school on Monday and then in parentheses Columbine," Lt. Craig McGuire with the St. Charles County Sheriff's department told the St. Louis Post-Disptach.

In 1999, two student gunmen went on a rampage, killing 13 people at Columbine High School in Littleton, Colorado.

In this instance, authorities discovered no evidence of an actual plot.

Word of the message first spread between students by phone, the Internet and text messages over the weekend.

A phone call to the district was not immediately returned Tuesday morning.

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Information from: St. Louis Post-Dispatch, http://www.stltoday.com