Spray cans have been found near where three men died after being struck by a train.
The incident occurred on tracks near Loughborough Junction station, south London, which is an area popular with graffiti artists.
British Transport Police (BTP) were called to the scene at 7.30am on Monday and rail industry sources told the Press Association the paint was found nearby and the three people may have been killed overnight, when freight trains operate.
Officers were seen taking photographs of graffiti as part of their inquiries.
The men are believed to have been in their 20s.
Officers have informed the family of one man and are working to identify the next of kin of the other two.
Superintendent Matthew Allingham told reporters at the scene that their injuries were "consistent with being hit by a train".
The first call to police was made by a train driver but the force is still trying to identify the train that hit the men.
There have been no reports of any passenger services being involved, which suggests they were struck by a freight train.
"We know they were dead for a while before we turned up," Mr Allingham said. "It is a possibility that it was during the hours of darkness. That's why it wasn't reported earlier."
The bodies were found on an elevated section of track with an electric third rail between Brixton and Denmark Hill.
Mr Allingham added: "There isn't a sort of safe refuge up there so if somebody was on those tracks there wouldn't be anywhere for them to go to avoid the train.
"As far as I understand it, if they've been caught in that section of track when a train came through then they really wouldn't have had much option."
BTP are treating the deaths as "unexplained" as officers work to understand "how these three people came to lose their life on the railway".
Anyone who was near to Loughborough Junction on Monday morning and saw something which may be relevant is urged to contact BTP.
Rail regulator the Office of Rail and Road has opened a preliminary investigation.