Police officers and staff who allegedly circulated an e-mail image of a decapitated black man are being investigated, it has been revealed.
The Independent Police Complaints Commission (IPCC) has been brought into the supervise the investigation, which involves Hertfordshire Police officers.
The force's own IT monitoring systems picked up the suspect e-mail.
On Wednesday Herts Police said its professional standards department would investigate the matter.
The e-mail reportedly contains a series of images, one of which shows the head of a black man who dies after jumping on to metal railings during a police chase. It is captioned: "Don't run from the police."
It is unclear if the e-mail is real or fake, but it is thought to have originated in the United States.
In a statement Hertfordshire Police said: "A wide-ranging and robust investigation is being conducted into the receipt, and subsequent distribution within the organisation, of a potentially offensive e-mail image.
"The inquiry was launched after an inappropriate image was sent to officers and staff within the organisation and, in a number of instances, distributed on by officers and staff to colleagues within the constabulary.
"The matter has been voluntarily referred to the IPCC, who have elected to supervise the investigation."
'No room for this'
Deputy Chief Constable Simon Ash described the e-mail attachment as "wholly inappropriate" and said it would not be tolerated.
"This e-mail attachment contravenes our organisational values and standards and there is absolutely no room in this constabulary for this type of communication," he said.
"There are a number of individuals who have allegedly forwarded this message to others and it is those officers and staff who are the main focus of our attention."
Det Sgt Alicia Moore, chair of the Hertfordshire Black Police Association, added: "The inquiry team has consulted closely with us since the beginning of this investigation and, to date, we are satisfied with the course of action that is being taken."