The infrared activity could mean there was a bomb on board or an engine exploded, according to two unnamed US officials.
US satellites picked up heat around a Russian passenger plane before it crashed in Egypt and killed all 224 people on board, two unnamed US officials have said.
One of the officials, who both spoke on condition of anonymity because they were not authorised to talk about the information publicly, said they had ruled out a missile strike as a possible cause because neither a launch nor an engine burn had been detected.
They said the infrared activity could mean many things, including that there was a bomb on board or that an engine exploded because of a malfunction.
Aviation analyst Paul Beaver said the heat detected “indicates that there was a catastrophic explosion or disintegration of the airplane”, but does not reveal what caused it.
Some experts have suggested a bomb was the most likely cause of the crash, while others have pointed to a 2001 incident in which the Metrojet plane damaged its tail during landing.
Militants from the Islamic State-linked Sinai Province have claimed responsibility, but the group has not provided further evidence and Egyptian President Abdel Fattah al Sisi has dismissed the claim as “propaganda”.
It comes as investigators prepare to start examining the black boxes retrieved from the crash site in the Sinai Peninsula.
The Russian aircraft’s cockpit voice recorder and flight data recorder are expected to shed light on what caused Flight KGL-9268, which was travelling from Sharm el-Sheikh to the Russian city of St Petersburg on Saturday, to break up near cruising altitude.
Egypt’s Civil Aviation Minister Hossam Kamal said a committee of international investigators will conclude its field inspection at the crash site on Tuesday before starting work on the black boxes.
Mr Kamal said it would “take some time” to produce the final report and that the committee “has all the tools and experts to deal with the investigation”.
Bosses from Russian airline Kogalymavia, which owns the Metrojet brand, have insisted only “some kind of impact” could have caused the disaster.
Egypt’s civil aviation ministry said on Tuesday there were no facts at this stage to substantiate assertions that the plane broke up in mid-air.
Spokesman Mohamed Rahmi confirmed that no distress call was received before the plane went down.
Alex Smirnov of the Russian Emergency Situations Ministry said 140 bodies and more than 100 body parts were taken to St Petersburg on Monday and Tuesday and that a third plane was set to bring more remains later on Tuesday.Families have identified the first 10 victims.