A first-year medical student was quietly studying in a public library when she was approached from behind and stabbed more than 20 times in what is believed to have been a random attack, police said.

Deane Stryker, 22, was pronounced dead at a local hospital over the weekend after she was attacked by Jeffrey Yao, 23, in Massachusetts' Winchester Public Library, authorities said. She attended the University of New England, where she was pursuing a medical degree.

Yao is being held without bail after pleading not guilty during a Monday court hearing.

"Jeff has a long history of mental illness, including multiple hospitalizations," Yao's lawyer, J.W. Carney Jr., said in court, according to the Boston Globe.

"This terrible tragedy, which shocked his parents, is unquestionably related to his severe mental issues," Carney said.

Prosecutor Kate Cimini called the incident "a vicious, unprovoked attack with a knife on a young woman in a public place." Yao, she said, had a history of run-ins with law enforcement.

 A 77-year-old man who tried to help Stryker was stabbed in the arm, police said. He was treated and released at a local medical center. 

Yao's weapon was a 10-inch hunting knife, police said.

The attack is one of several that have occurred at public libraries across the country, including a brawl captured on video after a teenager hit a security guard in a Baltimore public library, and an attack at a New Jersey high school.

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