Shoot-down hits US credibility. The loss of an aircraft to Nicaraguan gunfire raises new questions about `private' American mercenaries and their links to the CIA. It has also heated up the propaganda war between Washington and Managua.

The Reagan administration faces a growing credibility crisis over the downing of a privately chartered US cargo plane in Nicaragua. United States officials insist the plane, which was loaded with rifles, ammunition, and other military equipment apparently destined for contra guerrilla forces, had no connection to the US government. But there appear to be links of the aircraft, and its crew, to the Central Intelligence Agency, and two congressional committees are calling for an investigation into these links.

An American captured when Sandinista troops shot down the plane said yesterday he worked with CIA employees and took part on 10 such flights from Honduras and El Salvador.

Eugene Hasenfus said on a nationally broadcast news conference in Nicaragua that four of the flights were made from Aguacate air base in Honduras and six from Ilopango air base in El Salvador.

He said that from 24 to 26 ``company people'' assisted the program in El Salvador, including flight crews, maintenance crews and ``two Cuban nationalized Americans that worked for the CIA.'' Hasenfus said the CIA employees' jobs were ``to oversee housing for the crews, transportation projects, refueling and some flight plans.'' He said he was told he would be paid $3,000 per month plus housing and expenses for working with the air crews.

Congressional investigators say they'll be focusing on three questions: who authorized the ill-fated flight, how it got to Nicaragua, and whether anyone connected with the flight, including the crew or carrier managers, was involved with the CIA.

Meanwhile, State Department officials are debating their next move in response to threats by Nicaragua's Sandinista government to put Mr. Hasenfus, the lone survivor of Monday's air crash, on trial in Nicaragua.

Charges of possible illegal US involvement in the six-year contra war in Nicaragua -- and the credibility questions this raises -- come as the Reagan administration seeks to contain damage from disclosures that it allegedly launched a disinformation campaign designed to topple Libyan leader Muammar Qaddafi, including deliberate attempts to mislead the US press. On Wednesday State Department spokesman Bernard Kalb resigned in protest over the issue.

Under existing legislation, US intelligence agencies are barred from involvement in efforts to overthrow the Sandinista government. Congress is about to grant the CIA a major operational role in the contra war as part of a pending $100 million aid package to the rebels.

So far, none of the conservative private US organizations set up to abet the contra effort have claimed sponsorship of the downed plane. Retired US Army Gen. John K. Singlaub, widely regarded as the most prominent fundraiser and organizer for pro-contra efforts, has denied any knowledge of the plane or its mission.

Questions about possible CIA involvement in the flight have hinged on reports that two of four crew members of the plane had previously been employed by a Miami-based air cargo company, Southern Air Transport. Southern is one of several air-cargo companies once owned by the CIA, then divested after the Vietnam war. According to an April 1976 report of the Senate Select Committee on Intelligence, most were sold or given to ``witting individuals,'' including former officers, employees, managers, and contractors.

``In several cases, transfer of the entity was conditioned as an agreement that the proprietary would continue to provide goods or services to the CIA,'' the report continues. In 1973, Southern was purchased by Stanley Williams, who had run the company for the CIA for 11 years. Lawrence Houston, who was General Counsel of the CIA at the time of the sale, said that as far as he knew there was no continuing relationship between the CIA and Southern.

William Kress, a spokesman for Southern, insisted that Southern has ``no contracts with the CIA, nor have we since they owned the airline prior to 1973.''

But press reports have linked the company with the CIA. CBS News reported in 1984 that a CIA-sponsored Southern flight had landed in Honduras with 22 tons of small arms on April 9, 1983. Honduras has been the main supply line for supplies to the contras.

Southern has been linked to the Nicaraguan crash because of company identity cards found on two of the dead crew members. In addition, the plane had been serviced at Southern facilities in Miami. According to news reports, the C-123 cargo plane was purchased six months ago by a California company, Four Aces Inc., and subsequently leased to Southern Air Transport.

Questions about other possible CIA links to Southern have also arisen in the aftermath of the crash of a Southern cargo plane at Kelly Air Force Base in Texas early Saturday morning. Congressman Henry B. Gonzalez (D) of Texas is asking for a congressional investigation to determine if the CIA was using Southern to ferry arms and supplies to contra groups in Central America. Air Force and Southern officials have denied the allegations.

Manuel Cordero, a spokesman for the Nicaraguan Embassy in Washington confirmed Nicaraguan plans to put Hasenfus on trial saying ``we have to bring this situation to justice. We have to think in terms of the people who might have been killed or terrorized with the weapons that this guy helped deliver.''

State Department spokesman Charles Redman said Wednesday that Nicaragua's unwillingness to allow immediate consular access to Hasenfus ``has raised the question of whether indeed a US embassy can function normally in Nicaragua.'' The comment has been interpreted as a threat to quit the embassy in Managua or to sever diplomatic ties with Nicaragua.

Analysts say both sides may have reason to settle the question as quickly as possible. Holding an American captive and staging what would almost certainly be viewed in the US as a kangaroo trial would probably translate into stronger US support for the Nicaraguan resistance. Meanwhile, Reagan officials seem eager to have an embarrassing incident off the front pages.

In 1984 two private US citizens killed in the downing of a similar rebel supply flight over Nicaragua were returned by the Sandinista government within three days after routine processing and ID checks.

Even if a ``smoking gun'' is eventually found linking the latest downed flight with the CIA, analysts say, it is unlikely to have a dramatic political effect.

``It's a moot point, since the CIA is about to provide this stuff anyway,'' a congressional source says.