Officials are investigating a ransomware attack on the Los Angeles public school system. The Los Angeles Unified School District said the attack was “likely criminal in nature,” reports the Wall Street Journal. The public school system, with more than 500,000 students, is the nation's second-largest, after New York City. Los Angeles schools opened as planned on Tuesday, with no interruption to classroom instruction and transportation. The school district said there was “significant disruption to our system’s infrastructure.” Some business operations may be delayed. In a ransomware attack, malicious software is used to deny access to computer systems or data until a ransom is paid. Malware is often disseminated through email attachments or links that infect a device and its connected networks. After encrypting files, hackers demand payment of some kind of cryptocurrency in exchange for releasing the files and systems. Federal investigators and cybersecurity experts have been called in to work with the Los Angeles school district’s technology division, LAUSD said in the statement. Cybersecurity attacks increased during the height of the pandemic as classroom instruction went virtual. “The education sector has been heavily targeted for several years now,” said Brett Callow of the with cybersecurity company Emsisoft. He said 26 school districts were hit with a ransomware attack in the U.S. so far this year. Hackers released sensitive data from 24 of the districts.