SAN SALVADOR, JAN. 9 -- Leftist rebels in El Salvador admitted today that one of their units may have shot dead two injured U.S. servicemen after their helicopter was downed Jan. 2.

A rebel statement said two combatants had been detained "under the charge of suspicion of assassinating wounded prisoners of war."

The U.S. helicopter was downed in the eastern province of San Miguel as it was flying back to its base in Honduras from a visit to San Salvador. The pilot was killed in the crash, according to the Pentagon, but its autopsy team concluded that the other two Americans in the helicopter were killed execution-style afterward.

Civilians confirmed that the two servicemen had survived the crash, although none said he actually saw the shootings.

The Farabundo Marti National Liberation Front "has concluded that there are sufficient elements to presume that some of the three, in the condition of wounded prisoners, could have been assassinated by one or various members of our military unit," said the rebel statement. It also said their investigations had determined that their initial information from units on the ground was false.

At first, the guerrillas said the bodies of the Americans had been found in the helicopter. Then they said two of the three servicemen had survived the crash but later died of wounds. On Friday, an FMLN communique, citing "an exhaustive investigation," said, "It is totally false that our units shot and killed the injured crew emembers."

Salvadoran officials have said that if the Americans were executed, the guerrillas should hand over those responsible. The call was echoed by Rep. Joe Moakley (D-Mass.), the chairman of a congressional task force on El Salvador.

"We would expect and we would demand that the FMLN turn over to the judicial authorities those responsible. If not, this lack of action will have serious consequences," he said.

But the rebel statement made no promise to do that. "If responsibility for the crime is proved, the FMLN will act with all rigor, in conformity with our normal war justice," read the statement. The rebels said that because of the nationality of the victims, the investigations would be carried out publicly.

The rebels also defended shooting down the helicopter, which they said was flying in "attack position" in a conflict zone. The Huey helicopter is the same model as those used by the Salvadoran army and was flying very low to evade antiaircraft missiles.

A U.S. Embassy spokesman said officials are studying the rebel statement.

In the last few days, the rebels have sounded out United Nations officials about the possibility of setting up an independent commission, made up of U.S. and U.N. investigators as well as the FMLN, according to a senior rebel source.

The killings have opened a debate in Washington on whether $42.5 million in military aid to El Salvador, frozen by Congress last October, should be released. The money was withheld to protest the pace of investigation into the killing of six Jesuit priests by elite army soldiers a year ago.

{In Washington, knowledgeable sources said President Bush will report to Congress that the FMLN, by continuing to receive arms from outside the country and by attacking civilian targets, has violated the conditions placed by Congress when it withheld the aid. However, it was not clear whether or when Bush would decide to release that aid. His report to Congress, which was due Wednesday, has been delayed, sources said, apparently over concern that Congress should focus on legislation on the Persian Gulf.}

On Tuesday, two prosecution lawyers from the attorney general's office resigned in protest over what they described as lack of progress in the investigation of the Jesuits' death. The two lawyers earlier accused the army of a coverup to protect senior officers. They said the investigation carried out by Salvadoran investigators, with U.S. help, had never tried to uncover who ordered the killings.

Staff members of the Moakley task force also made new accusations of an army coverup in the case after a visit to El Salvador on Tuesday.

"The attorney general has failed to press the investigation against senior military officers. As a result, the boundaries of serious inquiry have been tightly controlled, and every effort to expand the circle of suspects has been contained by perjury, amnesia, obstructionism or silence on the part of the armed forces," said the statement.