In a letter to the university community, Loh apologized for the problem.
Loh said the hackers may have gained access to the names, Social Security numbers, birthdates and university identification numbers of 309,079 people who have been issued university IDs at the College Park and Shady Grove campuses since 1998. He said no other records are at risk.
The president said he was advised of the security breach late Tuesday. The university is working with law enforcement in an effort to identify the hackers, he added.
"Universities are a focus in today's global assaults on IT systems," Loh said. "We recently doubled the number of our IT security engineers and analysts. We also doubled our investment in top-end security tools. Obviously, we need to do more and better, and we will."