Police are concerned for the welfare of a woman filmed on a mobile phone being subjected to "a horrendous, prolonged attack" in a bath.
The mobile phone footage was circulated on BlackBerry Messenger (BBM).
The identity of the victim and where she was is not known but footage of the assault has been distributed in the Cardiff area.
South Wales Police says officers were tipped off about the attack by a concerned parent.
The video shows a young white woman in the bath, in pyjamas, being assaulted by a girl in dark clothing.
The suspect has dark hair tied in a bun and appears to be wearing distinctive false eyelashes and black and white patterned ankle boots.
A second woman watches the attack and also takes part in it.
Det Insp Shane Ahmed, of Cardiff CID, said: "In the video, this girl is being absolutely beaten in a bath.
"She is repeatedly punched, kicked, her neck is stamped on, her head is hit against the bath," he told Cardiff-based Media Wales.
"The attack appears to continue after she falls unconscious, with bathroom detergents poured over her," he said.
"It is a horrendous, prolonged attack and it is made even more worrying by the fact that we don't know who the victim is, where the attack has taken place or even if she is alive and breathing now.
'Identify the victim'
"We can tell from the labels on the products in the bathroom that this appears to have taken place somewhere in the UK, and we have contacted other forces to make them aware.
"Our priority is to identify the victim and ensure her welfare."
The video has been distributed via mobiles in the last 24 hours, added Mr Ahmed.
A 16-year-old girl arrested in connection with the incident has been released, a police spokesman said, adding: "It has been established that she is not one of the females in the footage and therefore no further action will be taken against her."
Anyone with information is asked to contact Cardiff CID on 029 2052 7420 or Crimestoppers on 0800 555 111.