A pit bull attack on the 4 train was sparked by a disagreement over a subway seat, according to witnesses. The incident, caught on video, has now resulted in a police investigation and dismay from the MTA.

"There’s no reason in the world why that dog was allowed on board, down to the platform, and on the train, let alone harassing one of my passengers," MTA Chairman Joe Lhota told the NY Post, explaining that the NYPD was looking into the assault. "Our system is open to everyone. It is not open to people with dogs that aren’t service animals or enclosed."

The dispute took place on Friday, April 20th, on a downtown 4 train around 3 p.m. According to WABC 7, Tahysi Kyng, who filmed the exchange, said when the dog and its owner entered the train, the pup "sat on my foot, and I really didn't mind it much."

When room opened up across the aisle, Kyng said the man sat down and put his dog on the seat. As the dog lay down, Kyng said the pet bumped a female passenger on the other side.noting that the female victim said to the dog's owner, "'The dog don't belong on the seat, that's an animal, people belong on the seat, put the dog on the floor.'.. And [the owner] looked at her like, 'I'm not moving my (expletive) dog.'"

From there, it went downhill quickly. Kyng said the woman first pushed the dog off the seat. The owner put him back up, and she shoved the dog off again. The owner responded with fists.

"He was like, 'Don't touch my dog,' and he started hitting," Kyng said. "They started fighting, and everybody tried to break it up. The dog latched onto her."

Kyng's girlfriend Denise Leon also witnessed the scene and described the events to WCBS 2, "[The dog's] jumping on her, but she moves back. That’s when he catches hold of her shoe and he was not letting it go." Leon also struggled with how to react, "At the same time, you want to help her, but you don’t want the dog to turn on you."

Kyng also noted that the pitbull's owner "never told the dog to let her go at all." Eventually the dog let go of the woman's shoe, and by the time the train got to Wall Street, the woman was able to walk off the car. She has not been identified, and it's unclear if she was seriously injured. The NYPD is searching for the dog's owner.

The MTA only allows dogs in carriers or service animals. The MTA's rules further clarify that "service animals" are "a guide dog, signal dog, or other animal individually trained to perform tasks for the benefit of a person with a disability that such person is unable to perform due to such disability, such as guiding persons with impaired vision, alerting persons with impaired hearing to sounds, pulling a wheelchair, retrieving dropped items or providing rescue assistance. The term service animal does not include a therapy animal or animal used for emotional support."

Summoness are given to owners who free-range their dogs on the subway, but an NYPD spokesperson characterized them as being "infrequently" issued: "Only 85 were issued in 2017, a tiny percentage of the 70,000 total summonses issued across the transit system last year," the NY Times reported this year.

Lhota urged riders to tattle on canines walking fancy free in the subway: "I encourage New Yorkers, if they see anything like that, to report it."