WOBURN, Mass. (AP)   A Massachusetts man charged with fatally stabbing a woman at a library during what prosecutors called an unprovoked attack has been held without bail.

Not guilty pleas to charges including murder were entered on behalf of 23-year-old Jeffrey Yao at his arraignment Monday.

Authorities say Yao stabbed 22-year-old Deane Stryker 20 times with a 10-inch hunting knife at the Winchester Public Library on Saturday. A prosecutor called it a “vicious, unprovoked attack.”

Yao’s lawyer says his client has “severe mental illness,” which includes multiple hospitalizations.

Stryker was a student at the University of New England’s College of Osteopathic Medicine who hoped to become a doctor.

Yao and Stryker attended Winchester High School at the same time, but it’s unclear if they knew each other and the motive remains under investigation.