The anniversary of the Columbine massacre is believed to be the inspiration for vague threats of violence circulated by Petaluma students on social media, a school district spokesman said Thursday.

An investigation is underway as to the source of the threats, first noticed Wednesday on Snapchat, said David Fichera, communications coordinator for Petaluma City Schools.

He did not provide more specific information about the nature of the threats.

But he said so far, no one has made good on them and authorities have been unable to link them to any student.

Police made a number of home visits Wednesday night to teenagers who had been involved in reposting them, he said.

And school administrators sent an email to parents of all high school students Thursday morning, alerting them to the investigation.

The massacre at Columbine High School in Colorado happened April 20, 1999. Two students killed 13 people before killing themselves.

Check back later for more details.

You can reach Staff Writer Paul Payne at 568-5312 or paul.payne@pressdemocrat.com. On Twitter @ppayne.