Detectives investigating the "brutal killing" of a father-of-two in his own home have found a knife near to the scene.
Robert Symons died in hospital
Robert Symons was stabbed in a struggle with a burglar on the ground floor of the house in Chiswick, west London, on Wednesday morning.
The 45-year-old teacher was found by his wife and died later in hospital.
Scotland Yard said a knife was recovered on Thursday morning and forensic tests were being carried out.
A police spokesman said they were hunting a man seen leaving the scene and believed the killer had previously tried to break into other houses nearby.
Pupils at the Queens Park Community School, north-west London, where Mr Symons worked as science teacher were being consoled by staff.
Headteacher Mike Hulme said: "The loss of human life in such circumstances is difficult to comprehend and all at the school are deeply saddened.
"Our thoughts are with his family at this time of their immense grief. He will be sorely missed."
Assistant Commissioner Tarique Ghaffur described Mr Symons' death as the "brutal killing of a decent man trying to protect his own home.
Detectives are hunting a man seen leaving the house
"We would like to reassure everybody that we are pulling out all the stops and will be doing everything in our power to ensure that the person responsible for this is captured in the shortest possible time."
Police said a man seen leaving the property was wearing a light-coloured short sleeved shirt and may have had an accomplice.
Mr Symons' wife Lin and their two-year-old and five-year-old daughters are with police liaison officers and will be interviewed about the incident later.
A post-mortem examination is due to be held at Fulham mortuary on Thursday.