A 13-year-old boy has been arrested on suspicion of murdering a man who died during a break-in, Scotland Yard said.
The teenager has been taken to a South London police station for questioning.
A 15-year-old boy has already appeared in court charged with murder while a 17-year-old has been released on bail after being arrested on suspicion of murder.
Martin Thomas, 39, was found with serious injuries by police and paramedics at his home in Huntingfield Road, Roehampton, south west London.An air ambulance crew arrived but he was pronounced dead at the scene.A post-mortem examination revealed he died from a stab wound that punctured his lung.
Following the stabbing Detective Chief Inspector Charlie King said: "At this early stage we are keeping an open mind about any possible motive.
"Members of the public have already provided us with accounts of three young men seen loitering in the area prior to the incident. I am directly appealing to these young men to come forward and speak us.
"I am also urging anyone who may have more information regarding this incident to contact us."