NEW YORK (CBSNewYork) — Police say they have made an arrest in a violent mugging on the Upper West Side that left an 87-year-old professor in critical condition.

Fifty-year-old Matthew Lee was walked out of the 24th precinct Wednesday night after police said he was arrested on charges of robbery and assault.

When asked by a reported, “did you do it?” Lee replied, “I don’t remember.”

The attack, which was caught on surveillance video, happened last Sunday at an ATM inside a Citibank on West 96th Street and Broadway.

The video shows 87-year-old Young Kun Kim standing next to the ATM. Seconds later, the suspect moves in, hitting him before pulling Kim out of the frame, CBS2’s Natalie Duddridge reported.

Police say the suspect punched Kim in the back of his head, critically injuring him, before taking off with his money.

The City University of New York’s website still lists Kim as a professor there. People who know were shocked by what happened.

“This is the most vulnerable person on the planet and you’re attacking him. For what? For some money,” said neighbor Eric Schwartz. “It’s just beyond.”

“He would come for breakfast or for lunch. Sometimes he’d meet friends,” said Rosa Soto, a waitress at the Metro Diner. “He comes here all the time, every single day. We just love him.”

Police said Kim was taken to the hospital “in extremely critical condition.” Sources tell CBS2 his condition has not improved and he’s on life support.

“That’s terrible,” said neighbor J. Randall. “I can’t actually believe that and now he’s still in the hospital?”

Police said Lee has one prior arrest for assault in 1984. He is expected to face a judge sometime Thursday.