Greece’s problems deepened on both sides of the Atlantic as the Federal Reserve disclosed it was investigating Goldman Sachs and other banks that helped the country mask its debts, and investors grew increasingly leery of lending any more money to a nation flirting with default.

On Thursday, the Moody’s ratings agency joined Standard & Poor’s in warning that it might downgrade Greek government bonds, a move that would increase the premium Athens must pay to borrow. The move comes at a precarious time for Greece, which must raise 25 billion euros ($34 billion) over the next few months to avoid a sovereign default that officials fear could cause the finances of other weak European economies to collapse.

In a sign of the challenges their nation faces, Greek officials also called off a planned trip to the United States and Asia aimed at interesting new investors in its bonds because of a lack of demand, according to an investment banker who was briefed on the government’s fund-raising strategy.

The European Union has said it would come to Greece’s aid only if it develops a plan to reduce its deficit by March 16, further ratcheting up the pressure.

“Even if they bring the deficit to zero, with interest rates at 6.5 percent and a growth rate of zero at best, Greece’s debt ratio remains on an explosive path,” said Miranda Xafa, a former executive board member at the International Monetary Fund. “I just don’t think they can raise funds from the market now.”

Greece has suffered from large deficits for years, and until now it seemed as if big banks would always be there to bail it out. As far back as 2000 and 2001, Goldman helped Athens quietly borrow billions to mask its poor finances by creating derivatives that essentially transformed loans into currency trades that Greece did not have to disclose under European rules.

Ben S. Bernanke, the Federal Reserve chairman, told Congress Thursday that the Fed was “looking into a number of questions relating to Goldman Sachs and other companies and their derivatives arrangements with Greece.”

Mr. Bernanke said the Securities and Exchange Commission was also concerned about how derivatives — financial instruments that are largely unregulated and do not trade on public exchanges — have contributed to Greece’s problems. “Obviously, using these instruments in a way that intentionally destabilizes a company or a country is counterproductive,” he said.

The S.E.C., in a statement, said that it could “neither confirm nor deny the existence of an investigation,” but added that it was cooperating with United States and international regulators in examining “potential abuses and destabilizing effects related to the use of credit-default swaps and other opaque financial products and practices.”

Goldman declined to comment, citing its policy of not addressing legal or regulatory matters. But in a Feb. 21 presentation, Goldman said, “The Greek government has stated (and we agree) that these transactions were consistent with the Eurostat principles governing their use and application at the time.” Eurostat is the European Union’s statistics agency.

Goldman is not the only bank that supplied derivatives designed to lower deficits. In the late 1990s, JPMorgan Chase helped Italy reduce its budget gap by swapping currency at a favorable exchange rate. In return, Italy committed to future payments that were not booked as liabilities.

A spokeswoman for JPMorgan said that Italy disclosed all of the deals to Eurostat.

Senator Christopher J. Dodd, Democrat of Connecticut and the chairman of the Senate Banking Committee, also took aim at credit-default swaps, which allow banks and hedge funds to wager on whether a company or country might default.

Critics say the swaps have contributed to Greece’s problems and increased the odds of a financial collapse.

“We have a situation in which major financial institutions are amplifying a public crisis for private gain,” he said.

The Fed inquiry was begun about three weeks ago, according to an official involved in the investigation who was not authorized to comment publicly. Fed examiners are focusing on whether Goldman and other banks complied with guidance the Fed issued in 2007 outlining