The State Department cautioned Romania yesterday to carefully handle a criminal investigation involving Rompetrol Group NV, a major oil refining and marketing company that includes a U.S. citizen among its top executives.

"We urge the Romanian government to observe due process, to be open and objective in handling the case and to ensure that the judicial process is fully transparent," State Department spokesman Tom Casey said.

Romania is actively seeking membership in the European Union by 2007, but the high-profile case threatens to raise questions about Romania's transition to a fully functioning judicial system. The State Department, in its most recent human rights report, cited continuing political influence over Romania's judicial system, while the American Chamber of Commerce in Romania has denounced the recent detention of the company's chief executive, Dinu Patriciu, for 16 hours on an array of charges, including money laundering and tax evasion. Romanian courts have twice rejected requests by prosecutors to authorize Patriciu's arrest.

Rompetrol Deputy Chief Executive Phil Stephenson, who served in the administration of President George H.W. Bush, is also under investigation, as is another U.S. citizen, former chief executive John Works, according to media reports in Romania. Rompetrol officials have denied wrongdoing and said the investigation was politically motivated.

President Traian Basescu ran for election on an anti-corruption platform, and he has lashed out against criticism of prosecutorial tactics, suggesting that a failure to pursue such investigations will set back Romania's bid for membership in the European Union. "If political parties, business lobby groups and the media rise against state institutions trying to deal with major corruption, thus hindering lawful actions of these institutions, then EU integration will certainly remain just a dream, not a reality," he said last week.

The Romanian Embassy in Washington declined to comment yesterday.

U.S. officials noted that the investigation appears to have started before Basescu was elected, suggesting not all of the allegations are politically inspired. Prosecutors have alleged the tax evasion cost Romania more than $10 million. But political influence over the judicial process remains a concern among U.S. officials.

"The Constitution provides for an independent judiciary; however, in practice, the judiciary remained subject to political influence," the State Department human rights report said. "Widespread corruption remained a problem, although the Government took initial, but only partial, steps to address the problem."