Police are concerned for the welfare of a woman filmed being assaulted in a bath in footage later circulated on BlackBerry Messenger (BBM).
The identity of the victim and where she was at the time is unknown but video of the assault has been distributed in the Cardiff area.
Officers were tipped off about the attack by a concerned parent who contacted South Wales Police.
They then viewed the video showing a young white woman in the bath, dressed in pyjamas, stood over and 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 is also present and watches the attack and also takes part in it.
"Such is the nature of such messages, we don't know where or when this assault took place," said Detective Inspector Shane Ahmed, of Cardiff CID.
"We are aware the footage has been distributed in the Cardiff area in the last 24 hours. Our priority is to identify the victim and ensure her welfare."
A 16-year-old girl arrested in connection with the incident has been released, a police spokesman added.
"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."
South Wales Police is also liaising with other police forces across the UK to establish if they are aware of the footage.
Mr Ahmed later described the assault as "an horrendous, prolonged attack".
"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.
"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.
"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."
Anyone with information is asked to contact Cardiff CID on 029 2052 7420 or Crimestoppers anonymously on 0800 555 111.