The Defense Ministry said the aircraft carried at least 50,000 rounds of ammunition for AK-47 automatic rifles, dozens of rifles, an unspecified number of rocket-propelled grenades and other equipment for the contras.

The CIA and other U.S. government agencies are currently barred from aiding the contras with military supplies, funds or advice. That prohibition will be lifted once Congress clears President Reagan's $100 million contra aid package. The legislation has been stalled since summer, but its approval was expected before Congress adjourns later this month for the fall congressional elections.

A contra spokesman in Washington, Javier Arguello, said the aircraft did not belong to the contras and that he did not know who those aboard it were.

Officials of the various private groups that have supported the contras also denied any link to the flight. Those included Alabama-based Civilian Material Assistance and retired Maj. Gen. John Singlaub, a principal private adviser to the contras. Singlaub was traveling in the Far East, but the executive director of Singlaub's Arizona-based U.S. Council for World Freedom, Joyce Downey, said "the plane is not ours."

'VERY BRAVE PEOPLE'

Elliott Abrams, assistant secretary of state for inter- American affairs, told reporters that private relief flights have enabled the Nicaraguan rebels to survive in the face of the restrictions on U.S. government assistance.

"Some very brave people . . . have been willing to actually bring this material into Nicaragua, as seems to be the case with this flight, " Abrams said. "All I can say for the people who were on it . . . God bless them, because they were fighting for freedom in Central America and keeping the option alive while Congress made up its mind."

In the small Wisconsin town of Marinette, population 12,000, many residents quickly recognized the name of Eugene Hasenfus, even though the Sandinistas first spelled it as Hafenfuf.

The name is well known in Marinette, a town near Michigan's Upper Peninsula, about 50 miles north of Green Bay. The Hasenfus family is large, has been in the area for almost a century and once owned a popular supper club.

Editors and reporters at the Marinette Eagle Star said they called the Hasenfus house, in a rural area outside the town, as soon as they heard the name on television and radio news.

Hasenfus' wife Sally talked to a few reporters but by Tuesday afternoon she referred all calls to her husband's brother William, a 47-year-old telephone salesman in Oshkosh, about 100 miles southwest of Marinette.

William Hasenfus said his brother Eugene went into the construction business after leaving the Marines, but then went to work for Air America as a civilian contract employee, along with his friend Cooper.

In the 1970s, the CIA divested itself from Air America but those who monitor American intelligence community affairs say the spy agency and the company continued to maintain informal contacts.

Working as a load master, responsible for weighing and properly balancing any equipment that is put aboard a cargo aircraft, Hasenfus flew missions for the CIA-linked firm throughout Indochina in the late 1960s, William said.

Returning to the United States after "two or three years" with Air America, Eugene Hasenfus went back to work as a construction worker and last summer, landed a job with a Florida air freight company, his brother said. William Hasenfus said he could not remember the name of the company.

He declined to say whether Cooper played a role in helping him get the job, but insisted that the CIA had nothing to do with it.

He said Eugene is probably alive because of a parachute he borrowed before going to Florida for his new job.

"That was my parachute on his back, " William said. "He asked for it when he visited a few weeks ago and that's what saved him." William said he and Eugene were both "ardent" skydivers and that while in Florida his brother had joined a skydiving club.

William described Eugene as having a "good old American build on him, " six feet tall and with red hair. He said Eugene has been an athletics enthusiast since high school and loves to have "beer with the boys" once in a while.

The downing of the aircraft Sunday came one day after a cargo plane owned by Southern Air Transport of Miami crashed at Kelly Air Force Base in Texas, apparently with cargo for the contras. Three crewmen were killed in the crash.

Rep. Henry Gonzalez, D-Tex., said in remarks from the House floor Monday that that plane was on a CIA mission for the contras. A CIA spokeswoman denied Gonzalez's assertion.

Sunday's incident is the first time the Sandinistas have captured an American who apparently was working for the contras.

A U.S. Army pilot, Chief Warrant Officer Jeffrey C. Schwab of Joliet, Ill., was killed Jan. 11, 1984, by Sandinista fire after his helicopter accidentally strayed into Nicaraguan airspace from neighboring Honduras.

Two other Americans, members of the Alabama paramilitary group CMA, died when their helicopter was shot down Sept. 1, 1984 during a contra attack.