A large number of US schools and universities or evacuated students Thursday and at least 20 people were arrested or under investigation as a wave of copycat campus threats spread after the Virginia Tech massacre, US media reported.

Phone, calls, messages posted on the internet, post-it notes stuck on lockers in the corridors, students carrying guns are just some of the many cases these days when copycats are trying to get the public attention.

The overwhelming majority of the threats referred to Monday's massacre in Blacksburg, Va., or the 1999 Columbine High School killings, authorities said. Friday is the eighth anniversary of the Columbine attacks.

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