In the post-Soviet era, Kazakhstan remained closely tied to Russia by energy supply lines, national defense, and the importance of Russian technologists in Kazakhstan’s economy, but Nazarbayev also sought closer relations with the West. Beginning in the 1990s, the discovery of major new oil fields and subsequent international investment enabled Kazakhstan’s economy to pull far ahead of its Central Asian neighbors. Since his first election in 1991, Nazarbayev has maintained firm control of Kazakhstan’s political and economic policy, removing all potential political rivals, including four prime ministers. A new constitution ratified in 1995 significantly expanded presidential power. After canceling the 1996 presidential election, in 1999 Nazarbayev easily won an election that received international criticism. However, by the mid-1990s the ruling elite already had begun to show signs of factionalism. Beginning in 1999, a series of corruption scandals arose, and frequent changes of government disrupted economic policy. In 2002 the government arrested the leaders of the top opposition group, the Democratic Choice of Kazakhstan (DVK), and pressured the media to stop critical reporting. In 2004 corruption allegations against the Nazarbayev regime intensified as a U.S. oil executive was indicted on charges of bribing representatives of the Kazakhstan government. In 2004 and early 2005, a series of restrictions on opposition parties, including the liquidation of the DVK, brought accusations of a political crackdown in advance of the 2006 presidential election.