NEW YORK (CBSNewYork) -- Another unprovoked attack on a train was caught on camera this week.
A Bronx man is facing charges after he allegedly assaulted a passenger.
It's still unclear what led to the incident, but police say a 62-year-old retired postal worker with Alzheimer's disease was randomly attacked as he rode the F train downtown Thursday morning.
The victim, who doesn't want to be identified, can't understand why he was singled out.
"I sit there, and somebody hurt me, hurt my head," he told CBS2's Valerie Castro.
The assault left him with a gash in the back of his head and other injuries. Most of the incident was captured on cellphone video.
The man recording begins frantically pushing the emergency button, calling for help, saying, "We need emergency assistance. This guy is tripping out here. He's hurting people, fighting."
Meanwhile, other bystanders try to intervene. You can hear what's believed to be the suspect's shrieks, but it's unclear why.
The train finally pulls to a stop at 23rd Street in Chelsea, and several people begin to follow the suspect, all the while yelling for law enforcement.
The man recording the video chased the suspect up a stairway and out to the street, but by the time he got there, the man had disappeared.
Police on patrol in the area eventually arrested the suspect -- 23-year-old Justin Pena, who has a prior arrest for assault.
Sources say he was taken to a local hospital for a psychiatric evaluation.
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