Police are searching South Norwood Lakes in Croydon as part of their Jamie Gilbey murder investigation.
They are asking anyone who might have been fishing or using the path around the lake to call police if they think they saw anything suspicious.
They also want to hear from anyone who saw any suspicious activity on roads near the lake, including Auckland Road, Sylvan Road and Woodvale Avenue.
It comes after officers on patrol on Sunday at around 1.45pm found suspicious items in Love Lane in South Norwood.
These were sent for further analysis.
A murder investigation was launched as it was thought that Jamie Gilbey, 20, has come to harm.
Police had previously appealed for information about the whereabouts of Jamie, from Crystal Palace, in a wanted appeal.
He had no prior convictions or cautions for any offence.
He was last seen on Sunday, January 27.
There have been no arrests.
Jamie's family have been informed and are being supported by specially trained officers.
Detective Inspector Melanie Pressley, from Specialist Crime, said: “This investigation is a terrible ordeal for the friends and family of Jamie and I would ask anyone who has any information, or may have witnessed suspicious activity, to call police.
"We also want to hear from anyone who has information that could help us piece together a picture of Jamie's life and the people he has been seeing. Your information, no matter how small, could be the vital help that ends his family’s anguish.
“I understand that the police searches will cause considerable concern to local people, but I want to stress that we are not aware of any wider risk to the community.
"This is a fast-moving and complex investigation, and we are following a number of positive leads as we work to establish what has happened to Jamie.
"Your information, no matter how small, could be the vital piece of information, so do call us, or you can phone Crimestoppers if you do not want to give your name.”
Chief Superintendent Dave Stringer, responsible for local policing in Croydon, Sutton and Bromley, said: “My officers remain on scene, providing a visible presence in the area for some days to come.
"I urge any members of the public who have any concerns, or information that could help us bring Jamie’s family justice, to approach those officers and speak to them.”
A Met Police spokesperson said: "Homicide detectives continue to appeal for information following the disappearance of Jamie Gilbey.
"In the course of the inquiry, specialist officers are currently carrying out searches of South Norwood Lakes in Croydon.
"Police are asking for anyone who may have been fishing or using the path around the lake to call police if they think they saw anything suspicious.
"They also want to hear from anyone who saw any suspicious activity on roads near the lake, namely: Auckland Road, Sylvan Road and Woodvale Avenue.
"At about 13:45hrs on Sunday, 27 February, officers on patrol discovered a number of suspicious items near Love Lane in South Norwood, these have been sent for further analysis.
"As part of our inquiry, it is thought that Jamie Gilbey, aged 20 and from the Crystal Palace area, has come to harm and a murder investigation has begun.
"His family have been informed and are being supported by specially trained officers.
"We had previously appealed for information about Jamie's whereabouts in a wanted appeal, but police stress he had no prior convictions or cautions for any offence.
"He was last been seen on Sunday, 27 January.
"There have been no arrests at this stage.
Contact the homicide team on 020 8721 4622 quoting Operation Wominghall with any information.
To remain 100% anonymous contact the independent charity Crimestoppers on 0800 555 111.
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