Rescue workers search the devastated hangar
A total of eight people are thought to have died in the accident at the Baikonur cosmodrome in Kazakhstan on Sunday.
The eight were repairing the roof of one of the hangars - used for assembling and testing space vehicles - when three out of five segments crashed 80 metres (260 feet) to the ground.
Local emergency services are said to have been prevented from entering the building, which has been cordoned off, because it was unstable and there were fears the walls might collapse.
Little hope
Sergei Gorbunov, spokesman for Rosaviakosmos, Russia's space agency, said it was unlikely that any of the missing workers had survived the fall.
"In the course of searching the debris, it has been possible so far to discover the fragments of the bodies of three fitters," he said.
Mr Gorbunov said the accident could have been prompted by something falling on a fuel tank kept inside the hangar, which would have produced a huge blast.
Space officials have ruled out terrorism or poor building maintenance as causes.
Shuttle damaged
Officials at Rosaviakosmos said three columns supporting the roof at building 112, a vast hangar used for Russia's "Buran" (Snowstorm) shuttle spacecraft, had given way.
One Buran spacecraft - reportedly the only one of the three built to have flown in space - was inside the building.
A 30-strong rescue team including seismologists, explosive experts and sniffer dogs has been sent to Baikonur from Moscow.
The BBC's Nikolai Gorshkov says Russia has been short of funds to maintain the complex, and parts of it have fallen into disrepair.
The Russian Aviation and Space Agency has set up a special centre to investigate the causes of the incident.
Financial difficulties
The Baikonur cosmodrome, Russia's main commercial launch site, was built in the 1950s and is the oldest working launch site in the world.
The first manmade satellite to orbit the Earth was launched from there.
The Buran project was initiated in 1976 in response to the US space shuttle programme, but was abandoned after the collapse of the Soviet Union. It completed one unmanned flight in 1988.
More recently, Baikonur has been used to launch commercial satellites and rockets bound for the International Space Station.
Kazakhstan took possession of the cosmodrome after the disintegration of the Soviet Union.
But it lacked the funds and expertise to maintain the site, which was looted by local people who sold sophisticated equipment for scrap metal, our correspondent says.
Russia has been leasing Baikonur from Kazakhstan since 1993, though the two former Soviet countries have on several occasions been at odds over rent payments and accidents during launches.