TV channel Five has been criticised by the advertising watchdog for sending out "seriously offensive" advertising material to homes.
CSI:NY stars Gary Sinise as a crime scene investigator
Up to 30,000 people received fake dossiers that suggested they were being stalked by a serial killer and included bogus photos of murder victims.
The grisly dossiers were promotional material for the crime show CSI:NY.
But 197 people complained to the Advertising Standards Authority (ASA) saying the package was "distressing".
The material was sent to homes in brown envelopes of the type used by investigators in the series, a spin-off from the highly successful CSI: Crime Scene Investigation series in Las Vegas, which also runs on Five.
The pack also featured pictures of forensic evidence from a crime scene and a wanted poster, which bore a large CSI:NY logo printed at the bottom.
It included macabre photographs, a coroner's report, an apparently bloodstained fabric sample and a "wanted" picture of a suspect dubbed The Carbon Copy Killer.
Fake newspaper cuttings described the murder of three people, all with the same name as the recipient, in New York.
Code of practice
Five said the mailing had been sent to 30,000 people selected from a database who had registered their interest in crime-related TV and had opted to receive relevant information.
But those who complained said it was not clear that the material was an advertisement.
Five said it had never intended to cause distress but claimed "a cursory review of the file made clear that it was marketing material".
The ASA ruled that the mailing breached its code of practice, but acknowledged the steps taken by Five.
The ASA said the "disturbing nature of the programme and contents of the mailing would seriously offend those who had not specifically requested information about the CSI series".
It concluded that the overall presentation of the mailing was "ambiguous" and likely to "confuse" recipients.