The death follows the murder of a former mayor and the stabbing of another acting mayor in the Siberian town, where the headquarters for Yuganskneftegaz, Yukos' former main asset, are located.
Acting Mayor Igor Gribanov died from breathing carbon monoxide fumes in a fire in his apartment on the night of Jan. 6, Nefteyugansk Deputy Prosecutor Igor Vavilin said by telephone.
He said a short circuit appeared to have sparked the blaze.
Gribanov declined further comment. He told RIA-Novosti, however, that prosecutors had opened an investigation, largely as a formality required by the law.
Gribanov took over City Hall after Acting Mayor Dmitry Yegortsev was stabbed in the hallway of his apartment building in September. Police have not identified a motive for the attack.
The town's last elected mayor, Viktor Tkachyov, was suspended by a court in February due to an investigation into whether he misappropriated municipal funds. Tkachyov was reportedly an ally of Yukos, which was stripped of Yuganskneftegaz over back tax claims in December 2004. The investigation into his activities is ongoing.
Gribanov appeared to be on friendly terms with Yuganskneftegas' new owners, state-owned Rosneft. In an interview on local television on Dec. 30, Gribanov described the town's relations with Rosneft "as between partners," according to the City Hall web site.
Mayor Vladimir Petukhov was killed in an apparent contract hit in 1998. Weeks before he was slain, he had demanded that Yukos pay alleged tax arrears and had led a demonstration of municipal workers that disrupted a meeting of Yukos shareholders in the town.
Last July, the Prosecutor General's Office charged former Yukos security chief Alexei Pichugin with organizing the killing. Pichugin, who has denied wrongdoing, was convicted in March on separate murder charges and is currently serving a 20-year sentence in prison.
The Prosecutor General's Office has also charged core Yukos shareholder Leonid Nevzlin with involvement in the mayor's killing. Nevzlin, who has denied wrongdoing, lives in Israel.
News agency Regnum, citing an unidentified source, speculated Tuesday that Gribanov might have been killed on orders from rivals ahead of a possible mayoral election. No election can be held unless Tkachyov is found guilty of misappropriating the funds.
The head of the town's legislature, Rashid Tagirov, rejected the speculation. "It's nonsense. The election has not even been called yet," he said by telephone.
Carbon monoxide was blamed for the deaths of Georgian Prime Minister Zurab Zhvania and his friend Zurab Usupov in Usupov's Tbilisi apartment on Feb. 3. Investigators determined that the gas had leaked from an improperly installed heater and said the apartment lacked proper ventilation.
In another attack in Nefteyugansk, town legislator and Yuganskneftegas deputy director Sergei Burov was shot and wounded in the stomach in May.
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