Schools in Haverhill, Massachusetts, were closed Thursday after the district's computer system was hit with a ransomware attack.
The Haverhill Public School District said its IT department noticed early Wednesday morning that something was wrong with the system, shutting down the network "before large scale corruption of the system occurred."
The attack, designed to cripple a system until money is paid, takes times to resolve, cyber security experts say.
"They will attempt to look at the signature or the symptoms of the ransomware that they've been impacted by to identify what may be the way to unlock it or reverse it back," said Scott Margolis, founder of SolutionLab, LLC.
Fixing the issues can be costly, too, according to another official.
"Ransomware opens up this really interesting moral dilemma, especially if you're an organization that is tax funded," said Katie Ledoux, an information security professional.
The school's entire system, including its remote learning platform, is not operational, the district said.
Thursday was supposed to be the first day back in the classroom for students in grades 2 through 4.
"It's not fair to the kids," said Carolyn Camacho, a parent of a student. "They should be able to just go to school and not have any issues whatsoever."
The district says it is considering Thursday a snow day and will make it up on Friday, June 18.