A U.S. locust-spraying plane was shot down near where Marxist re-bels have been fighting Morocco for an independent Western Sahara, and officials said all five people aboard were presumed killed.

A second DC-7, also from the U.S. Agency for International Development, was damaged in Thursday's anti-aircraft missile attack over Mauritania but was able to land at a remote airstrip in Morocco. No one aboard that plane was injured.Marxist guerrillas of the Polisario Front may have fired at the four-engine propeller-driven planes, official sources in this North African country said on condition of anonymity.

The guerrillas are known to have been armed with Soviet-made SA-6 and SA-7 surface-to-air missiles. Both the Moroccan and Mauritanian armed forces also have such anti-aircraft missiles.

No one immediately claimed responsibility, and a U.S. Embassy spokesman in Rabat said an investigation was under way.

"The five crew members of the downed aircraft, all of whom are American citizens, are presumed dead," an AID statement from Washington said.

Both aircraft were owned by T & G Aviation of Chandler, Ariz., and were under contract to AID to spray insects, said T & G co-owner Sergio Tomassoni, who was in the other plane that was hit.

Tomassoni, 64, spoke with reporters by telephone from his hotel room in Agadir, Morocco.

He identified those aboard the downed plane as Joel Blackmer, 46, and Blackmer's son, Frank Kennedy, 21, both of Phoenix; Ben Rossini, 49, Tempe, Ariz.; Frank Hederman, 47, Cody, Wyo.; and Wes Wilson of Nebraska. Tomassoni did not know Wilson's age or hometown.

Tomassoni described the two-hour trip in the crippled plane to the next airfield as "hairy."

"We were just flying along at about 11,000 feet when, all of a sudden, the first airplane was hit," Tomassoni said. "We saw the smoke and a big ball of fire. One of the engines was in flames. The plane started losing altitude, and then the right wing blew off. I knew they were in trouble, but we had problems of our own."

"We were 25 to 30 feet above the ground most of the way. We had a load of insecticide aboard, and we dumped it and were able to gain some altitude," Tomassoni added. "We made an emergency landing at an airfield in a town about 80 miles south of Agadir."

A caller identifying himself as Madjid Abdullah, Polisario's North American representative in Washington, said the front "will do everything possible to shed full light on this tragic and unfortunate incident."

"We are not sure really who shot it down," he said. "The Polisario Front has no interest in creating problems with the United States."

He said Morocco must assume responsibility for the attack because it directed civilian planes over an area with of Polisario troops.