Los Angeles Unified School District
Schools in the Los Angeles Unified School District close after receiving an unspecified "electronic" threat. Later, authorities conclude the threat was a hoax, and announce schools will reopen Wednesday. New York City schools' initial response to a similar threatening email was to treat it as a hoax. The emails were largely identical and had been routed through a server in Frankfurt, apparently by the same person.
Sources: BBC News CNN ITV LA School Report The New York Times