President Boris Yeltsin declared Monday a day of mourning for the victims of the blast and of the Sept. 4 car bomb that demolished an apartment building in the southern Russian region of Dagestan, killing 64 people.
That blast was blamed on Islamic militants battling Russian troops in the region -- and many Muscovites, including Mayor Yuri Luzhkov, assumed they were behind the Moscow explosion, as well.
The blast in Moscow on Thursday had a force equal to about 440-660 pounds of TNT. Investigators were concentrating largely on two possible scenarios: a terrorist bomb or an accidental detonation of explosives stored in the building.
The Federal Security Service, which is heading the investigation, made a preliminary assessment that the blast was caused by "an industrial explosive or a pyrotechnic mixture," according to spokesman Sergei Bogdanov. He did not say whether the blast was considered accidental or premeditated.
The cause that was originally suspected, a gas leak, was hardly mentioned on Friday. Most officials appeared to be leaning toward the likelihood that it was a bomb, though Prime Minister Vladimir Putin said the cause was still under investigation.
"If it was a terrorist action, we're facing a sly, treacherous and bloodthirsty enemy," he said in a televised address. Putin tried to reassure Russians, who have been shaken by a series of explosions and unnerved over the war in the south.
Luzhkov said Friday that traces of the explosive substances hexogen and trotyl were found at the site and were likely planted in the office of a company called Delko-2, on the first floor of the building. He said the premises had long been empty and the owners of the company were being questioned.
Police were searching for a man who sublet the Delko-2 office, state television reported.
A hard-line Communist leader alleged that the blast was linked to the political feud between Luzhkov, the Kremlin and other forces. Luzhkov is considered a strong favorite to succeed Yeltsin after presidential elections next year, and he has accused Yeltsin and his circle of being jealous of his popularity.
"Political hysteria is being fanned artificially, including by way of explosions, to cancel parliamentary and presidential elections through a state of emergency," Viktor Ilyukhin was quoted as saying by the ITAR-Tass news agency.
He called on Luzhkov, who also heads the Fatherland national political movement, to drop out of the political competition.
Moscow police stepped up security in metro stations and other public places, and swept apartments for alleged criminals from the Caucasus.
Some newspapers and television on Friday showed a composite sketch o a man in glasses who was seen near the building shortly before the explosion.
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