Police appear to be powerless to bring charges against people who distribute flyers warning potential informants against helping detectives, one of Britain's top officers admitted.
Scotland Yard is trying to find those behind "Stop Snitching" leaflets posted on an estate where a teenager was shot dead.
But the acting head of the Metropolitan Police conceded that his officers were likely to do little more than talk to the people responsible for the flyers.
Acting Commissioner Tim Godwin said: "I think it would be quite difficult to identify criminal offences within it. There may be stuff we can do in civil law, but I think it's more about having a chat with them. We need to identify them to have a little discussion."
The leaflets urging people to refuse to co-operate with the authorities were posted on the Pelican Estate, in Peckham, south-east London, where 17-year-old Sylvester Akapalara was killed last month.
Investigators offered potential witnesses protection and anonymity if they came forward in the hours after the attack.
Sylvester died after being chased into a stairwell in Heron House. Three teenagers have since been charged with murder.
Mr Goodwin said the Met's relationship with London's communities had improved "dramatically" over the past decade and people were now more willing to come forward with information.
"I think if people are then coming out and saying, 'please don't talk to the police, they are the enemy', that is because they now feel threatened," he said.
"So I see that as a positive sign, that we've got them worried. The individuals concerned need to be dealt with."