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The blast hit the 10-story apartment house in the industrial city of Dnipropetrovsk, about 600 kilometers (370 miles) from Kiev, on Saturday, destroying much of the building and ripping off parts of the roof and blowing out dozens of windows.
Four people were still missing and were being sought under the debris, the Emergency Situations Ministry said in a statement. Among the 21 confirmed killed are seven children.
Prosecutors believe the blast occurred because the valves that regulate gas pressure in pipes had worn out.
Two employees of the local gas operating company have been detained pending potential negligence charges.
Natural gas explosions resulting from improper usage, poor infrastructure or faulty butane and propane tanks are common in the former Soviet Union.
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