The scenario leading to the 1976 bombing assassination here of former Chilean ambassador Orlando Letelier became more confused yesterday as a paid FBI informant told a federal court jury a version of the events he said had been related to him by one of the Cuban exiles charged with the killing.

The informant, Ricardo Canete, said that the Cuban, Alvin Ross Diaz, told him he had used a "timing device" to make the bomb that killed Letelier as he drove along Washington's Embassy Row on Sept. 21, 1976.

In addition, Canete recalled that Ross told him that the bomb was placed in Letelier's car in a Washington garage and that some of the alleged conspirators in the case stayed at an Arlington motel while they were here carrying out the Letelier assassination.

The testimony of Canete, who has been convicted on larceny and counterfeit money charges, conflicts with that of the government's key witness, American-born Chilean secret police agent Michael V. Townley. Townley testified earlier that the bomb was detonated by radio control, that he placed the bomb in Letelier's car as it was parked in the driveway of Letelier's Bethesda home and that the alleged conspirators stayed in District of Columbia hotels.

Canete conceded under cross-examination that in mid-1977, when Ross told him he "made Letelier's bomb... I did not believe Mr. Ross was giving me accurate information."