Internet rumors about the threat of a shooting at Selma High School kept nearly one-third of students home Tuesday, authorities said.

Though the threat turned out to be a hoax, about 500 of the campus' 1,700 students were absent after the rumors spread on Facebook over the past few days, Selma Unified Superintendent Mark Sutton said.

Police and school officials investigated the rumors and visited the homes of students implicated in the Facebook postings about 7 a.m. Tuesday, Sutton said. The students and their families said they knew nothing about the threats and were cooperative with officials, he said.

In fact, the rumor stemmed from a threat made by a student more than a year ago that was handled at the time, Sutton said.

Nonetheless, he said, "We can't afford not to take things like this seriously."

School officials dispelled the rumor in an automated phone message sent about 10 a.m. Tuesday to every parent in the district.

"No Selma schools have received any recent threats of any kind," the message said.

The message encouraged parents to send their children to school.

Absences at Jackson Elementary School, which is adjacent to Selma High, also were higher than normal, Sutton said. He did not have specific information about the number of absences.

No arrests have been made and police have not been able to find out who was the source of the Facebook posting about the threat, police Chief Myron Dyck said.