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It was an unexpected attack inside the community college’s Rockville campus library. The victim told police on Monday that she had been followed by her attacker and then she saw him again that very night in the library.

Minutes later, she told police he had her cornered inside the bathroom. What she didn't know is that another student heard her cries for help.

“Someone would have to be horrified to make that kind of sound,” said Alexander Cahan.

For several seconds, he heard screaming and crying coming from the women's bathroom on the second floor of the library.

"I thought maybe someone was crying on the phone, yelling on the phone, but it went on for about five or six seconds, so I thought something was definitely wrong,” Cahan said.

He was sitting at a table finishing a drawing when he got up and made his way to the bathroom door. Inside, the victim and the alleged aggressor stood still.

“I asked her if she was okay, did he put his hands on her,” said Cahan. “Then she couldn’t answer because she was in shock.”

Before she could answer, Cahan said the suspect, identified as 46-year-old Chong Yi, took off.

“He ran towards the exit, but he went out the wrong side,” said Cahan. “The door wouldn’t open, so I caught up to him, grabbed his jacket and slammed him to the glass.”

Cahan yelled for security as Yi got away from him. A second chase ensued and Cahan pinned him to the ground.

“It was obvious they didn’t know each other,” he said.

Not only did they not know each other, but police say Yi is not a student at Montgomery College.

The victim told investigators Yi followed her into the restroom, blocked her escape, touched her and made sexual hand gestures before pushing her back into the restroom.

“My heart broke for her when I saw her face,” said Cahan. “You can tell her innocence was gone after that. She was horrified.”