Police are trying to establish whether three men found dead in a shipping container in Berkshire were overcome by fumes from a generator.
Forensic police officers have been examining the container
It is thought the men, aged in their late 20s to early 30s, were working in the construction industry and had spent the night in the container.
Thames Valley Police said their deaths were being treated as unexplained and a full investigation was under way.
The bodies were found on a building site in Whitley, Reading.
Thames Valley Police said officers said one line of inquiry involved a generator found with them on Friday morning.
The alarm was raised when employees turned up for work at the former Thames Water treatment works site.
Graham Groves, of South Central Ambulance Service, said: "A paramedic team were on the scene within about six minutes of that call being received.
"Sadly all three males, aged in their late 20s to early 30s, were already dead and there was nothing further that the paramedics could do."
He added there was no obvious sign of injury to the three men.
Thames Valley Police said officers were liaising with the Health and Safety Executive (HSE).
An HSE spokeswoman said: "Obviously the HSE is concerned with the safety of employees at work but at this early stage I can only say that we are looking at the scene with police."
The Whitley building site is close to the old Thames Water treatment works, home of the new Southside development.
The Housing Corporation awarded almost £3m funding for developers to provide 60 affordable homes at the Reading Gateway/Southside development earlier in the year.