School officials in at least five cities received threats late Wednesday similiar to those targeting districts in New York City and Los Angeles earlier this week.
The latest threats singled out San Francisco, Miami, Fort Lauderdale, Houston and Dallas. All school districts determined the threats were not credible and remained open Thursday, two days after a similar threat prompted Los Angeles to shutter all 900 public schools in the country's second-largest school district.
Two school districts in Indiana — Plainfield and Danville — closed Thursday after also receiving threats overnight. It's unclear whether the threats in Indiana are linked to those affecting schools in other states.
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Authorities said the message sent Monday night to Los Angeles school board members — an email written from someone purporting to be a high school senior — described bombs hidden on multiple campuses. The threat said armed jihadi fighters would storm the schools.
It was later deemed a hoax, along with a nearly identical threat made to New York City schools, which remained open Tuesday as the LA district closed.
Miami-Dade County Public Schools and the Houston Independent School District announced on their websites Thursday that school officials had received a "less-than-credible" threat late Wednesday evening but would remain open.
Miami school officials said district police immediately contacted federal, state and local law enforcement agencies and promised to deploy extra security to schools Thursday.
In Houston, officials said law enforcement officers were making sweeps of school district buildings to ensure student safety. They encouraged parents and students to be vigilant and report any suspicious activity to police.
Officials from Broward County Public Schools in Fort Lauderdale said they received similar threats. In a tweet sent Thursday, the district announced that all schools would be open Thursday morning as scheduled.
Staff members at an elementary school and high school in Dallas also received threatening emails. Dallas schools remain open Thursday after authorities swept the schools with bomb-sniffing dogs and found no cause for concern, officials said.
School officials in multiple districts have said the threats all appear to be similar. Authorities are still working to track down the person or people responsible.
It's rare for a major U.S. city to close all its schools because of a threat. The move reflected lingering unease in Southern California following the attack that killed 14 people at a holiday luncheon two weeks ago in San Bernardino.
Campus safety officials predict the number of school threats related to terrorism will continue to rise.
Ari Mason contributed to this report.