A man has been defecating in trains across south-east England, causing damage costing £60,000 to repair.
Police have released a CCTV image of the suspect
British Transport Police have released CCTV images of the man, who has struck on at least 30 trains since August.
He waits until he is alone before committing the offence, smearing excrement inside carriages.
"This is a serious public health issue as well as being exceptionally anti-social - we need to locate this man," said Det Con Donna Fox.
She said his offences had resulted in many carriages being taken out of service, causing disruption and cancellations and serious inconvenience to the travelling public.
There was also £60,000 in damage and cleaning costs.
"There is no particular pattern as to when he appears," said Ms Fox.
"He travels to various areas and at different times of the day and different days of the week.
"We have been trawling through CCTV images to try and track the man and remain hopeful that members of the public may know him and where he lives.
"On at least one occasion CCTV footage shows the man being disturbed by a passenger walking through a train.
"If anyone sees this man travelling on the railway network they should not approach him, but call the police or alert train staff immediately."