Several teenage girls seen in a viral video attacking another group of teens aboard a Philadelphia subway train during an incident that was motivated by race are expected to be charged with several offenses, authorities said Thursday. 

The attack occurred around 3:30 p.m. aboard a SEPTA train headed southbound from the suburb of Olney. Video footage posted to social media shows a group of female teens yelling and berating another group of teens as they are sitting down. 

"This was an attack based on ethnicity, and ethnic slurs were used by the attackers," SEPTA Transit Police Chief Thomas Nestel told reporters Thursday. "It was clear that they were picked on because they're Asian."

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SEPTA train at a Philadelphia station. A group of students were targeted by another group Wednesday aboard a SEPTA train in an attack caught on video, police said. 

SEPTA train at a Philadelphia station. A group of students were targeted by another group Wednesday aboard a SEPTA train in an attack caught on video, police said.  (GETTY)

He said the attack was unprovoked. Fox affiliate WTXF-TV reported that the girls initially targeted a group of Asian males on the train car. They allegedly hurled racial slurs and dumped a smoothie on one of them, the news station said, citing sources. 

Nestel said there were four female suspects – who are African American – and four victims – three males and a female. 

"I'm appalled by yesterday's assault involving students on the Broad Street Line. Those involved in the assault have been identified and will be held accountable. I want our residents to know that we will not tolerate any acts of hate," Philadephia Mayor Jim Kenney said.

In the video footage, a girl is seen trying to intervene before she is thrown to the floor and hit several times. One attacker appears to use her shoe to beat the defenseless teen. 

Nestel said SEPTA patrol officers learned of the attack when the female victim approached them and reported that she and others had been assaulted in a train car. She was taken to a hospital to be treated for a laceration.

"She was a hero," Nestel said. "She stepped up and told the girls to stop saying what they were saying. She then became a target."

During the investigation, detectives checked subway cameras and images of potential suspects were sent to officers on patrol. Within an hour, two suspects were stopped by an officer.

Since they could not be identified as the suspected attackers at the time, the officer got their names, the school they attend and spoke with their parents before they were released pending further investigation. The mother of one of the other suspects reported her daughter as one of the attackers, Nestel said.

The four suspects range in age from 13 to 16 and are expected to face a number of charges, including ethnic intimidation, aggravated assault, simple assault, recklessly endangering another person, and terroristic threats, Nestel said. Affidavits were being drawn up Thursday, he added.

A train moves along the Market-Frankford Line in Philadelphia, Wednesday, Oct. 26, 2016. With a strike threat looming for Philadelphia's bus, trolley and subway workers next week, officials are asking customers in the nation's sixth-largest transit system to start figuring out alternate ways to get to work and school. The current contract covering more than 5,700 workers at the Southeastern Pennsylvania Transportation Authority expires at midnight on Monday, and a walkout could begin at the start of service on Nov. 1.(AP Photo/Matt Rourke)

A train moves along the Market-Frankford Line in Philadelphia, Wednesday, Oct. 26, 2016. (AP Photo/Matt Rourke)

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As a precaution, he said an officer will be waiting at the train station where the attack occurred to escort students.