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An independent public commission to investigate the bombings chaired by Duma deputy Sergei Kovalev was hampered by government refusal to respond to its inquiries , and its chairmen admitted that he has no evidence to support any version of the events .
 Sergei Kovalev
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Sergei Kovalev