Sponsors of an experimental spacecraft designed to use light from the Sun to power space travel have conceded that the mission is probably lost.
Hopes are "slim" that Cosmos-1 made it into orbit
But they said the apparent detection of signals from the craft by tracking stations remained to be explained.
The privately funded Cosmos-1 craft was launched on Tuesday on a Russian rocket from a submarine in the Barents Sea.
Russian officials said the modified missile carrying the craft failed during firing of its first stage.
Despite announcements by the Russian navy and space agency that the vehicle had been lost, the Cosmos-1 team held out hope throughout Tuesday that the launch had succeeded.
However, that hope has now all but faded. In a statement, mission sponsors the Planetary Society accepted the Russians' conclusion was probably correct.
But, it continued, "there are some inconsistent indications from information received from other sources".
Apparent signals were detected by three ground stations, at Petropavlovsk in Kamchatka, Majuro in the Marshall Islands and Panska Ves in the Czech Republic, the Planetary Society said. It added that doppler data was also detected over one of these stations.
But Jiri Simunek, a scientist at Panska Ves, told the BBC News website that no signal had been detected by the Czech tracking station, "just noise".
Nevertheless, Cosmos-1 scientists said there was still a slim possibility that the craft made it into orbit, though a lower one than expected.
"The project team now considers this to be a very small probability. But because there is a slim chance that it might be so, efforts to contact and track the spacecraft continue," said the Planetary Society statement.
A spokesperson for the Russian space agency said the Volna rocket booster carrying the spacecraft had failed 83 seconds after launch due to a problem with the first stage engine of the three-stage booster.
"The booster's failure means that the solar sail vehicle was lost," said agency spokesperson Vyacheslav Davidenko.
"The Russian navy is searching the area for the debris of the booster and the vehicle."
The Russian-built Cosmos-1 was launched aboard a modified Volna intercontinental ballistic missile (ICBM) from the nuclear submarine Borisoglebsk.
The $4m (£2.1m) experimental craft uses "solar sails" for power.
The sail reflects particles of light, or photons, from the Sun, gaining momentum in the opposite direction.
Some think solar sails offer a cheaper, faster form of spacecraft propulsion.
"Solar sailing is really the only known technology that could potentially take us to the stars one day because it does not have to carry fuel with it and because it can keep accelerating - even at incredible distances," the Planetary Society's Amir Alexander told the BBC.
The acceleration from sunlight is very small; but the advantage of solar sailing over chemical propulsion is that the acceleration is sustained.
Cosmos-1 would have got faster and faster - and climbed higher in orbit - as time went on.
The 100kg (220lbs) craft had been scheduled to reach an 800km- (500 mile-) high orbit.
It was then to have taken pictures of Earth for four days before unfurling its eight aluminium-backed plastic sail blades into a 30m (100ft) circle.
The US, European, Japanese and Russian space agencies also have solar sail programmes in the offing.