Cyber-attackers are believed to be targeting school districts in the New Jersey county of Somerset.
Security incidents that occurred in the county over the past week caused day-long school closures at two educational establishments. Schools in Bernards were closed on April 7 and Hillsborough schools were shuttered on April 12 following suspected cyber-attacks.
Computer systems and staff voicemail at Hillsborough remained down on Tuesday. However, students and staff have been able to access lessons virtually.
"Our technology team continues to work methodically with cybersecurity experts and law enforcement to establish a timetable for the completion of the restoration of the operation of our systems," Hillsborough superintendent of schools Lisa Antunes told parents in an email dated April 13.
She added: "This is an overwhelmingly complicated task with many moving parts."
While the investigation into the incident is ongoing, the superintendent was unwilling to give away many details.
"We are very limited in sharing the many factors that affect information we can provide to staff and families," said Antunes. "A timetable for the completion of the restoration of the operations of our systems cannot be provided at this time."
She added that the decision to close the school on Monday had been taken on the advice of the FBI and law enforcement "in an effort to minimize damage to the organization," opening up the possibility that the district has been hit by ransomware.
"Several hours ago, our technology department was alerted to a possible cyber-attack," wrote Antunes in a message posted to the school's Messenger page.
"Law enforcement, including the FBI, recommended shutting our systems down so that we may be able to ascertain the depth of the attack."
Bernards schools superintendent Nick Markarian told parents that the security incident affecting the district was "server focused." He said that individual Chromebooks, laptops, smartphones, and other devices connected to the district's network had not been compromised.
Markarian added that "additional protections will be added to all district issued devices in the coming days to monitor their health in new ways" as a precaution.