Iranian officials said today they discovered the hidden garage with the seven trucks that were to be used to carry an assault team to the U.S. Embassy here on a mission to rescue the American hostages.

The garage was located about 50 miles southeast of Tehran and the commandos were to hide there during the day before leaving at midnight for the drive to the embassy.

The April 24 rescue mission was called off when three of its helicopters broke down. Eight Americans were killed in an accident during the pull-out from a desert refueling spot.

The new information on the American rescue attempt was presented to the conference on U.S. intervention in Iran and was profusely illustrated with maps, satellite photos, charts and pilots' notes detailing many previously unconfirmed aspects of the plan. They showed a picture of the trucks lined up in front of the garage, which the Iranians said was owned by a CIA agent.

Many of the pictures and documents were interpreted for the Iranian government by Lt. Manouchehr Soltani, 27, an Iranian helicopter pilot who said he had been trained at four U.S. military bases.

"I am famliar with all of these," he said, pointing to a display case containing coded descriptions of landing sites, radio frequencies to be used during different parts of the mission and detailed descriptions of the helicopter flight path from carriers in the Indian Ocean to deep within Iran.

The display even included a receipt lost by one of the members of the American team for a gold necklace he purchased; a mimeographed invitation to a June 7 shipboard party that was to feature the burning of an Iranian flag, and a Visa card from a Virginia bank.

There was also a large satellite photograph dated March 31 of the desert refueling spot and a pilot's strip map illustrating the full flight path across Iran and showing the spot where the helicopters were to be hidden for a full day waiting to pick up the assault team inside Tehran.

The map had a footprint in the middle of it, apparently made by someone in a hurry to get away from the desert landing site.

The captured documents also said jet fighters from American carriers in the Indian Ocean would take off to escort the transport planes carrying the rescued hostages out of Iranian airspace. But there was no documentary information that they would be used in tactical support of the rescue mission in Tehran.