(Reuters) – U.S. law enforcement officials on Friday were probing a wave of hoax emailed bomb threats demanding bitcoin payment that caused worry but no damage in the United States, Canada, Australia and New Zealand a day earlier.

Hundreds of businesses, government offices and schools received the awkwardly-worded letters, threatening to set off explosives if payments of $20,000 in cryptocurrency were not received, sparking scattered evacuations of schools and transit facilities before the Federal Bureau of Investigation and other agencies dismissed the threats as lacking credibility.

Hoax threats were received on Thursday in cities including Washington, New York, Detroit, San Francisco, Los Angeles, Phoenix, Oklahoma City, Grand Rapids, Iowa, Denver, Ottawa and Calgary, Alberta.

Officials provided no new details on the investigation into who was behind the threats early Friday.

A similar wave of emailed hoax bomb threats in December 2015 prompted officials in Los Angeles to close the city’s public school system, which national law enforcement officials later criticized as an over-reaction.

Two weeks previously, a married couple inspired by Islamic State had killed 14 people at a California county office building in a shooting rampage.

A teenager with dual Israeli-U.S. citizenship was arrested in Israel in March 2017 for making bomb threats to more than 100 Jewish organizations and Jewish community centers in dozens of U.S. states over several months.