MANAGUA, NICARAGUA, DEC. 8 -- Sandinista forces shot down a small plane that violated Nicaraguan airspace Sunday and captured its American pilot, a farmer from Illinois who owns a ranch in Costa Rica, Defense Minister Humberto Ortega said today.

Gen. Ortega said authorities found documents in the plane linking the pilot, James J. Denby, 57, of Carlinville, Ill., to the Nicaraguan rebels, known as contras, and to the U.S. Central Intelligence Agency. He said Denby, an avowed contras supporter, had requested but not received permission to fly over Nicaragua.

Among the documents shown to reporters at a news conference was a 1984 issue of the American magazine Farm Journal, containing a photograph of Denby in an article about American ranchers in Costa Rica who were helping the contras.

Ortega, the brother of Nicaraguan President Daniel Ortega, said Denby was in Managua undergoing interrogation by state security agents. Sandinista officials at the news conference showed reporters a videotape of Denby, but did not make good an earlier pledge to present him to the press today.

{A contra spokesman denied that Denby was involved with the rebels, Washington Post staff writer Joe Pichirallo reported from Washington, and U.S. officials say that from all indications he is not involved with U.S.-sponsored operations that support the contras.}

The American, a portly man wearing a flowered Hawaiian shirt, was shown being led by Sandinista troops with his hands tied behind his back after his capture. He appeared to be in good physical condition.

Ortega said members of the Sandinista Navy hit Denby's small Cessna 172 with rifle fire at about 3 p.m. Sunday as it flew low over Nicaraguan territory north of San Juan del Norte, a port a few miles north of the border with Costa Rica. He said bullets hit the plane's fuel tank and Denby then had to make a forced landing on a beach on Nicaragua's Caribbean coast.

Documents found in the plane "confirm he is linked with the North American government through the mercenary forces," Ortega said. He implied that Denby might be put on trial on charges of having violated Nicaraguan airspace and "the security of the country."

Ortega added, "It's evident that he wasn't Santa Claus."

The Nicaraguan defense minister said that planes such as Denby's -- described as a 1950s vintage -- had often been used in the past for observation or spy flights on behalf of the contras. But he dodged a question on whether Sandinista authorities believe Denby was actually engaged in such a flight when he was shot down.

Ortega said Denby had taken off from La Ceiba, Honduras, on the flight that took him on a north-to-south route over Nicaragua without waiting for an answer from Sandinista authorities.

Friends and relatives of Denby's said he was flying his plane back from the United States to his ranch in Costa Rica, located in a remote area near the border with Nicaragua.

Denby has told reporters in the past that he helped the contras, sometimes flying wounded rebels to Costa Rican hospitals in his plane. He has denied running guns to the contras or profiting from the help given.

Ortega said Denby was a friend of John Hull, an American who owns a ranch near Denby's in Costa Rica and who has been linked with past contra support activities run by Lt. Col. Oliver North.

Ortega said that among the documents recovered from the plane were color negatives showing "mercenary camps and mercenary helicopters." He said there were also notes, addresses and phone numbers, business cards, flight books, maps and a license from the Illinois government that permits Denby to buy explosives.

In addition, there was a letter from Rep. Richard J. Durbin (D-Ill.) identifying Denby as a constituent. Ortega said the documents identify Denby "as an anti-Sandinista Republican and an anticommunist." He added, "All this proves the deep ties with North American aggression."

In interviews with reporters in Illinois earlier this year, Denby said he bought the 700-acre ranch in northern Costa Rica in 1973 to raise beef cattle, but that most of his stock was killed or eaten by the contras and Sandinistas after civil war broke out in neighboring Nicaragua six years ago. He said he then took to cutting timber and exporting orchids.

He boasted in the interviews that he was acquainted with some rebel leaders and CIA officials and that he had used his plane on occasion to ferry contra wounded to Costa Rican hospitals for free.

"I've helped them in any way they've asked me to," Denby told The Associated Press in August. "We feel it's our war, too." His remarks suggested that his involvement with the contras for the most part occurred around 1984, when U.S. aid to them was cut off.

Pichirallo, of The Washington Post, reported the following:

Bosco Matamoros, a contra spokesman in Washington, said, "We have no relationship with this gentleman."

Matamoros said clandestine flights that ferry arms to rebel troops fighting inside Nicaragua are all flown by Nicaraguans and no U.S. citizens are involved. The contras also do not have any Cessna 172 planes, he said.

The State Department and Central Intelligence Agency, which has the prime responsibility for overseeing U.S. military assistance to the contras, declined to make any official statements on whether Denby had ties to the U.S. government. U.S. officials who asked not to be identified said that all indications are that Denby was not part of current CIA operations to ferry weapons to the contras.

Denby is a friend of Hull, as mentioned by Ortega. In a telephone interview from Costa Rica, Hull said he has known Denby for about a dozen years but said that Denby "never helped me in any way, shape or form with any contra activity."

Hull said Denby was scheduled to meet a reporter and photographer from the St. Louis Post-Dispatch in Costa Rica. He said they had planned to fly with Denby from Belize but they could not fit on the plane because of fuel tanks Denby was carrying for his long flight.

Denby also owns a farm in Carlinville, a small town in southern Illinois, where he lives with his wife, Marie, and son, James Jr., most of the year, a relative said.