An American among hostages taken into Beirut from a TWA jetliner hijacked in June 1985 testified Tuesday that Moslem radicals threatened to put them back on board and blow up the plane. Allyn Conwell, testifying at the trial of confessed hijacker Mohammed Ali Hamadi, also said he was convinced he would be shot when he and others were initially taken from the plane. Hamadi, a Lebanese Shiite Moslem, is charged with murder and air piracy in the hijacking during which U.S. Navy diver Robert Stethem was killed and 39 Americans were held hostage for 17 days. Hamadi has confessed to the hijacking but denies killing Stethem. Conwell, a 43-year-old Houston executive, said the last 39 American hostages aboard the seized TWA jetliner were taken off the plane while Hamadi and his accomplice were sleeping. He said four American servicemen earlier had been removed from the plane by Hezbollah, a radical Shiite Moslem group backed by Iran, while the remaining Americans were in the custody of Amal, a more moderate Shiite militia. As delicate negotiations for the hostages' release ran into snags, Amal officials told Conwell ``the original hijackers were getting very upset over the lack of progress in negotiations,'' he testified. ``I was told the intention was to return all the hostages to the plane and blow it up,'' Conwell said. The hostages were shifted around different locations in Beirut until their release, he said. Conwell, who said he had been so ``terrorized that I was virtually immobile'' after the plane was hijacked on a flight from Athens to Rome, told the court that conditions improved for the hostages once they were under control of Amal. ``I was under the impression that Amal absolutely did not approve of the hijacking and the murder of Stethem,'' he testified. ``I think they acted to the best of their ability in a chaotic situation.'' During a break in the court proceedings, jailers could not free Hamadi from the court security room in which he is locked and had to use a cutting torch to open the door. Hamadi was arrested at Frankfurt Airport in January 1987 after customs officials found liquid explosives in his luggage.