An elderly man died from injuries sustained during a robbery at an Upper West Side bank on Sunday. The suspected mugger reportedly claimed he has amnesia and "bad spirits."

Young Kun Kim, 87, had been in the ATM vestibule of the Citibank on Broadway near 96th Street on the afternoon of May 13th when he was approached from behind by a man who punched him in the head. Police say that the suspect then took money from Kim and fled on foot.

WANTED: Male, Black, 45-50 yr old, wearing blue cap with headphones for assaulting an 87 yr old male victim inside Citibank at 2650 Broadway on Sunday May 13th at approx. 4:50pm. Please help us ID him. Call 24 Pct's Detective Squad at (212) 678-1863 with any information. pic.twitter.com/KXmpqioBfC — NYPD 24th Precinct (@NYPD24Pct) May 15, 2018

Kim was taken to Mount Sinai Medical Center, where he was put on life support. According to WABC 7, "Kim was known to eat at the same Upper West Side diner every morning before heading to work as a political science and Asian studies professor who has taught at Columbia University, Hunter College and CUNY."

Former student Ariana Collado told the NY Post Kim was even teaching in his 80s, "He was definitely sweet. I would say toward the end, it would honestly take him like five minutes to walk down the hall [because of his age]. But he came in, he did what he loved every single day... He was very understanding and willing to meet with any of the students if they needed too."

"This is the most vulnerable person on the planet. And you're attacking him for what? For money. It's just beyond. So I hope they find somebody," a neighbor lamented.

On Wednesday, Matthew Lee, 50, was arrested and charged with robbery and assault. When he was being escorted by police, Lee told reporters, "You have to talk to my lawyer. Something is not right. Bad spirits or something."

Now it's likely that Lee will face murder charges as well.