Howard University, one of the largest historically Black schools in the United States, canceled classes Tuesday after a ransomware attack, reports NBC BLK.

The attack shut down the campus Wi-Fi, and nonessential employees were instructed to not report to work, In-person classes resumed Wednesday (Sept. 8), but online classes remain canceled until at least Thursday.

Ransomware attacks have become a scourge in recent years, and universities are common targets. Hackers have managed to infect at least 19 colleges and universities in 2021 alone, according to Allan Liska, an analyst at the cybersecurity company Recorded Future. In at least three other instances in the U.S. this year, schools had to cancel classes for at least one day.

Criminal hackers use ransomware to break into a victim’s computer networks to lock up files on as many computers as possible and then ask for money for a code to restore them.

Many ransomware hackers also threaten to publish files they’ve stolen in the effort. So far, the university has seen “no evidence of personal information being accessed or exfiltrated,” Howard said in an announcement. Its investigation is ongoing, however, and identifying the full extent of what the hackers have done can take weeks or months of analysis.