Police are investigating after an image appearing to mock the death of striker Emiliano Sala was shared online.
Sala, 28, had just signed for Cardiff from Nantes when the plane he was travelling in crashed into the English Channel, north of Guernsey, on January 21.
A mocked-up boarding pass for Sala appeared online ahead of Sunday's derby clash between the Bluebirds and south Wales rivals Swansea at the Liberty Stadium.
In the image, described as "disgraceful" by Swansea, the boarding pass has the flight number "D3AD".
The flight captain is named as Mikey Dye, a Cardiff fan who died after an attack outside Wembley in 2011.
A spokeswoman for South Wales Police said: "South Wales Police is aware and have reviewed an image on social media relating to Emiliano Sala and we are investigating in relation to an offence under the Malicious Communications Act.
"We will be working closely with both Swansea City and Cardiff City football clubs to identify the source.
"We would remind people not to share the image as they may be committing an offence."
The force asked anyone with information to contact 101 or Crimestoppers on 0800 555 111, quoting occurrence 1900395865.
In a statement, Swansea said: "Swansea City is aware of the disgraceful image currently circulating on social media ahead of Sunday's South Wales derby in relation to the sad death of Cardiff City striker Emiliano Sala.
"We are working closely with South Wales Police who will be conducting an investigation to identify the initial source.
"The image is shameful and does not represent this football club or our supporters in any way."
