Ukrainian investigators suspect that Transport Minister Heorhiy Kyrpa - found dead with a bullet wound to the head - may have been driven to suicide.
Kyrpa was a key member of the ruling political clan
The Ukrainian prosecutor-general has launched a criminal investigation into the death of Kyrpa, 58, a key ally of Prime Minister Viktor Yanukovych.
Kyrpa was found dead in his holiday home outside Kiev on Monday, as the opposition celebrated its election win.
At one time Kyrpa was named as a possible presidential candidate.
Viktor Yushchenko, a pro-Western liberal, emerged as the clear winner in Ukraine's presidential election on Monday, defeating Prime Minister Yanukovych.
The investigation into Kyrpa's death was launched under the article "driving someone to suicide" in the Ukrainian criminal code.
Ukrainian news agencies quote unnamed security officials as saying Kyrpa was alone in the house at the time. They also allege that 100 minutes before he was found, he had a telephone conversation with an unknown interlocutor.
Mr Kyrpa was an important member of outgoing President Leonid Kuchma's entourage. He often accompanied the president on foreign trips and on holidays.
During the election campaign the opposition accused him of arranging transport to Kiev for Yanukovych supporters based in the eastern Donetsk region. The government denied bringing in the pro-Yanukovych miners, who staged brief protests in the capital before returning home.
Kyrpa became transport minister in July, having been nominated by Viktor Medvedchuk, the Social Democratic Party leader and presidential administration chief.
But Kyrpa never joined any parties and was believed by experts to be a fairly independent figure within President Kuchma's political clan.
His railway reforms earned him a reputation as an uncompromising manager who did not fear conflicts with the powerful steel and coal tycoons in eastern Ukraine.
But recently he had been accused of haste in preparing the sale of a 43% state stake in the national telecoms monopoly Ukrtelecom.
Under pressure from the opposition - who claimed the government was seeking to pay off lobby groups for their support in the election - the sale was cancelled.