The four people seen on CCTV footage during a barbaric attack on a woman in the Berlin metro are Bulgarian nationals, German prosecutors say.

All the suspects are thought to be relatives, Germany's Focus Online quotes an investigator as saying.

Earlier reports had only suggested the attacker himself comes from Bulgaria. While he is thought to have returned to his homeland, German authorities are preparing a request for his extradition back to Berlin, according to daily 24 Chasa.

The footage shows a group of four people walking past the steps that lead to a metro platform in Berlin Neuköln. One approaches a 26-year-old woman and kicks her in the back as is going down the steps and walks away.

The woman falls off the stairs, with later reports indicating she broke her arm. Another is seen walking to the site where the attacker dropped a beer can and picking it up without rushing to help the victim.

Posted on Facebook and YouTube, the video generated millions of views, drawing attention from German media outlets and the public.

One of the others was detained on Monday but was subsequently released.

The attacker himself, named Stanislav Stoykov, 26, lives in the Black Sea city of Varna's Roma ghetto of Maksuda and has a substantial criminal record of hooliganism, robbery and other offenses, private NOVA TV broadcaster says.

According to German authorities, the incident took place on October 27. However, it was only on November 17 that the victim filed a complaint with police.

Mass-circulation Bild newspaper soon published the CCTV footage, which was soon posted on YouTube.

24 Chasa's Berlin correspondent says that the metro station in question is known as an area known for restaurants and pubs that offer Bulgarian dishes and music.

In the European public space, the act of violence was used by both far-right groups and refugees, with the former (including in Bulgaria) blaming the incident of "refugees" and the latter arguing neo-Nazi groups were involved.

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