The Air Accident Investigation Board (AAIB) said Sala was exposed to harmful levels of carbon monoxide before he was killed and it is likely his pilot was also affected.
Tests on the striker's body found enough evidence of the harmful gas to cause a heart attack, seizure or unconsciousness, and it was ‘likely’ that pilot David Ibbotson was also ‘affected to some extent’ by the gas, they added.
The AAIB said the gas can ‘reduce or inhibit a pilot's ability to fly an aircraft’.
Argentinian footballer Sala signed for Cardiff City from French club Nantes for £15 million on January 18.
Mr Ibbotson, 59, of Crowle, Lincolnshire, flew Sala from Cardiff to Nantes the following day. The return flight - which crashed in the Channel - was on January 21.
Sala's body was recovered on February 6 but Mr Ibbotson has not been located.
The aircraft remains underwater off the coast of Guernsey after an attempt to recover it was hampered by bad weather.
Mr Ibbotson's widow Nora believes that trying to safely bring the aircraft up from the seabed could be a way to find some answers about what happened.
She also told Sky News that carbon monoxide potentially having any part in what happened had ‘never even occurred to me, it was all about the weather and things like that’.
She described the development as ‘a massive shock’, telling Sky News: "It makes a big difference because they've been poisoned, they have no idea it's a lethal gas.
"You can't smell it. You can't see it. It's lethal, they wouldn't have known.
"So it's nothing to do with the flying or anything like that, it's down to the aircraft."
Mrs Ibbotson said she must now wait for answers, adding: "It's not going to change my situation, I've lost a husband, a father, a grandfather. Nothing can repay that, and change it."
Piston engine aircraft such as the Piper Malibu involved in the crash produce high levels of carbon monoxide, the interim report said.
The gas is normally conveyed away from the aircraft through the exhaust system but poor sealing or leaks into the heating and ventilation system can enable it to enter the cabin.
The AAIB said it was working with the aircraft and engine manufacturers and the National Transportation Safety Board in the US ‘to identify possible pathways through which CO (carbon monoxide) might enter the cabin of this type of aircraft’.
"Work is also continuing to investigate pertinent operational, technical, organisational and human factors which might have contributed to the accident," they said.
A spokesman for Cardiff City said the club was ‘concerned’ at the AAIB's latest report as it ‘once again highlights that the aircraft used for Emiliano Sala was not appropriate’.
He added that ‘those who were instrumental in arranging its usage’ should be held to account.
Several devices are available to alert pilots over the presence of carbon monoxide.
The AAIB said they are not mandatory but can ‘alert pilots or passengers to a potentially deadly threat’.
Sala's family called for the wreckage to be salvaged to enable a ‘detailed technical examination of the plane’ to be carried out.
But an AAIB spokesman said it has decided not to carry out a repeat attempt due to the high costs involved, the information already collected and the risk that the wreckage would ‘not yield definitive evidence’ due to the plane's violent impact with the sea.
He added: "We will identify the correct safety issues through other means."
Investigators have previously said the validity of Mr Ibbotson's licence will form a key part of their inquiry.
The type of licence he held meant he could only fly passengers in the European Union on a cost-sharing basis, rather than for commercial flights.
A full accident report is expected to be published next year.