Lecturer was 'locked inside flat by his killer'
Bill Coventry, who lived upstairs from Roger Gray in one of Edinburgh's most upmarket suburbs, said he was unable to get inside his neighbour's flat even though he had a key.
He said Mr Gray's flat appeared to have been locked from the inside "like Fort Knox".
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Hide AdPolice are investigating whether the attack on Mr Gray, who was gay, may have been linked to his sexuality.
One theory being explored is that the 64-year-old may have known his attacker. Mr Gray was a senior lecturer at Heriot-Watt University for more than 40 years.
His body was found by police responding to reports of a gas leak and it is understood the gas supply in his home had been tampered with.
There has been speculation that the killer attempted to trigger a gas explosion to try to destroy the crime scene, then fled from a window in the flat.
Mr Coventry, 77, who lives in the Merchiston Crescent flat above Mr Gray, raised the alarm on Saturday morning when he smelled gas from his bedroom.
He said: "I had a key to Roger's flat because of a flood and because he didn't have any family.
"I went up and got the keys to Roger's door - a mortice and a Yale - and it wouldn't open, so there must have been an additional lock behind the door.
"I certainly couldn't get in."
Mr Gray retired from full-time work about four years ago, but still taught part-time at the university. He had travelled widely, lecturing in countries including Kenya, Hungary and China.
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Hide AdHe was a member of the Scottish Arts Club, the Edinburgh Festival Society, the Wagner Society of Scotland and the Scottish Actuaries Club.
Lothian and Borders Police say Mr Gray appeared to be the victim of a "sustained attack".
A spokeswoman for the force last night described any reports of how Mr Gray's killer entered or left his home as speculation.