Russia's top investigative body says no traces of explosives have been found in the rubble of an apartment building that collapsed this week, killing 39 people.

A section of the 10-story building in Magnitogorsk, about 1,400 kilometers (870 miles) southeast of Moscow, fell into a vast heap of jagged concrete on Monday after an explosion. Officials say a gas leak likely triggered the blast, although unconfirmed reports in Russian media have suggested it could have been an extremist act.

Investigative Committee spokeswoman Svetlana Petrenko said Friday according to state news agency Tass that "investigators and experts have completed their examination of the parts of the rubble taken from the apartment block complex. No traces of explosives or their components were found on the fragment."