U.S. lawmakers are considering tightening anti-corruption laws after a Senate report this month found that a prominent Washington bank helped top officials of the West African nation of Equatorial Guinea steal hundreds of millions of dollars in oil revenues.

The Senate Permanent Subcommittee on Investigations has been probing the corruption case involving Equatorial Guinea and its oil revenues for much of the past year.

The panel's report found that Riggs Bank helped government leaders in Equatorial Guinea siphon oil revenues to accounts set up for them in Washington.

The West African nation, where significant amounts of oil were discovered in the 1990's, has been cited by the U.S. State Department for human rights abuses, corruption, and diversion of oil revenues to government officials.

Beginning in 1995 until earlier this year, Riggs oversaw as many as 60 accounts containing as much as $700 million, making Equatorial Guinea its largest single customer. Some were government accounts, while others were the private accounts of President Teodoro Obiang Nguema, other government officials, and their families.

The Senate report says millions of dollars in the government accounts, which should have gone to help impoverished Equatorial Guineans, were instead funneled to off-shore tax shelters, with help from Riggs officials.

At a hearing on the matter at which current and former bank officials appeared last week, Senator Carl Levin, a Michigan Democrat, was clearly outraged. "Somehow there has to be a conscience here. Aren't you troubled?," he said.

The report describes one incident in which the bank manager of the country's accounts, Simon Kareri, brought a 27 kilogram suitcase full of cash into Riggs to make a deposit into President Obiang's account.

Mr. Kareri, who was fired in January, refused to testify at last week's hearing," he said. "Mr. Chairman, there is nothing I would like to do more than answer your questions today. However, I must heed the advice of my counsel and invoke my fifth amendment rights under the Constitution and refuse to answer the question," he said.

Senators were angered that bank officials never reported any suspicious financial transactions involving Equatorial Guinea.

Lawrence Hebert, president and chief executive officer of Riggs Bank, blamed a lack of an internal system to monitor and identify such suspicious activity. It is an assertion that Senator Levin found ridiculous.

"Without the system, without the upgrade compliance department, we would have been unable to produce that information and provide the necessary oversight," he said. "That is what we were working with the Office of the Comptroller of the Currency on."

"First of all Mr. Hebert, you do not need a computer system to realize suspicious activity when you have sixty pounds of cash that are being walked into the door with a suitcase," he said.

Mr. Levin criticized bank regulators for not doing enough in their oversight responsibility.

Riggs was fined a record $25 million by federal banking regulators for allegedly failing to report suspicious transactions made to the Equatorial Guinea accounts, but that didn't happen until May of this year.

Senator Levin also took aim at oil companies that are doing business in Equatorial Guinea, many of which have contracts with firms that have ties to President Obiang. The senator did not mince words in his questions to oil executives who appeared before his panel.

"Does it trouble you that you have a business partner like this dictator?," he asked.

Steve Guidry, leader of the Central Africa Business Unit at the Marathon Oil Company in Houston, Texas, made no apologies for his company's work in Equatorial Guinea.

"We feel that our presence in the country goes a great distance toward improving conditions in Equatorial Guinea, and we feel through our presence, to the extent that we have influence, we think we can have a positive effect on the conditions that exist in Equatorial Guinea," he said.

Mr. Guidry, and executives from Exxon Mobil and Amerada Hess, all said they complied with U.S. anti-bribery laws and have been involved in only legitimate business ventures in the West African nation.

But the non-profit group Global Witness, which probes links between the exploitation of natural resources and the funding of corruption and conflict, argues otherwise.

The London-based organization notes that the Senate committee report found that oil companies made payments into the personal accounts of Equatorial Guinean officials that were used for land purchases, office leases, and even education for the children of the country's leaders.

With oil money stashed away in Riggs Bank for the ruling elite of Equatorial Guinea, argues Sarah Wykes of Global Witness, oil companies cannot make the case that they are a force for positive change in the country.

"Equatorial Guinea is now the third largest producer of oil in sub-Saharan Africa," he said. "It has been called the 'Kuwait of Africa' But it is clear that since oil came on stream, the human development indicators of the country have actually gone backward, so we can say the oil money is not contributing to development at all," she said.

Ms. Wykes endorses the Senate report's call on Congress to strengthen the Foreign Corrupt Practices Act to require that U.S. oil companies disclose substantial payments they make to foreign governments, or to business ventures with links to foreign government officials or their family members.

"We need a level playing field where companies are going to be protected from having to do business with corrupt regimes in order to get a secure supply of oil," she said.

Ms. Wykes also calls on U.S. lawmakers to put pressure on the U.S. Justice Department to investigate the allegations in the Senate report and bring those involved in looting the assets of the people of Equatorial Guinea to justice.

The chairman of the Senate Permanent Subcommittee on Investigations, Republican Senator Norm Coleman of Minnesota, is considering doing just that.