It was about midnight when the Chilean secret police agent slid under the parked car of former Chilean ambassador Orlando Letelier outside his Bethesda home and attached a bomb, according to court testimony yesterday.

For three days the car traveled around the Washington area with the bomb fixed to the car body, an FBI agent testified, until it was detonated by a remote control device on Sept. 21, 1976, as it traveled around Sheridan Circle in the city. Letelier and an associate, Ronni K. Moffitt, were killed.

portions of the bomb - the blasting cap in particular - were brought into the U.S. from Chile by the secret police (DINA) agents and the bomb itself was actually put together in a Washington area motel with TNT brought here from New Jersey by the bombers, according to court testimony.

Two anti-castro Cuban exiles from northern New Jersey helped in the bombing, and one of those two actually denotated the bomb, according to FBI special agent L. Carter Cornick.

All of it was part of a DINA mission - complete with prebombing shadowing of Letelier to fix his movements - to silence the outspoken critic of the Chilean military regime headed by Gen. Augusto Pinochet, the agent testified.

The testimony came during a hearing in New Jersey to determine whether there is sufficient evidence to bring one of the Cuban exiles, Alvin Ross Diaz of Union City, N.J., to Washington to face conspiracy charges in the bombing case.

U.S. Magistrate Serena Perretti found the evidence supported the transfer and ordered Ross sent to Washington. Cornick's testimony is the first detailed accounting by a government official of the alleged bombing plot.

Another FBI agent testified that "electric matches," the portions of hlasting caps brought to the U.S. from Chile by American-born DINA agent Michael Vernon Townley, had been found in an apartment once occupied by a man identified as Ross in Union City. Townley, named as the agent who actually placed the bomb, has positively identified the "matches" as being among the same ones he brought from Chile because of the manner in which he modified them, the agent added.

Although yesterday's hearing focused on Ross's role in the alleged conspiracy, it provided more information than has previously been made public about the specific roles of the five other persons charged so far.

It did not include any detailed testimony on the manner in which DINA allegedly ordered the plot, nor disclose in any detail what Chilean government officials could be criminally culpable. Government sources have said in the past that at least three Chilean government officials could be charged in the alleged plot, including the now-ousted head of DINA, Gen. Manuel contreras Sepulveda.

The only Chilean discussed at yesterday's hearing was Capt. Armando Fernandez Larios, who has previously been identified as the second DINA agent who came to the U.S. as part of the alleged DINA mission to murder Letelier. The U.S. cannot attempt to extradite Larios, a Chilean citizen, until formal charges are filed.

Cornick, under questioning by Assistant U.S. Attorney Eugene M. Proper of the major crimes division in Washington, said the joint FBI-U.S. attorney's office investigation "revealed that two representatives of DINA traveled to the U.S. to arrange and facilitate the assassination of Orlando Letelier," and that they specifically were "on assignment for DINA to kill" the former ambassador.

As developed under the questioning by Propper and Ross defense attorney Paul Golberger. Cornick gave the following chronological version of the alleged international "hit" carried out by DINA in Washington:

Townley, who is cooperating with U.S. investigators and has agreed to plead guilty to one count of conspiring to murder Letelier, arrived at John F. Kennedy International Airport in New York on Sept. 9, 1976. There he was met by Fernandez, who had been following Letelier around for 10 days to ascertain his work and driving habits.

Townley then contacted Virgilio Paz Romero (who has been charged in the case and is a fugitive), whom he knew previously - apparently when Townley lived in Miami a few years ago. Paz, who now lives in New Jersey, and Townley met in the Newark area and the DINA agent asked to be put in contact with other members of the New Jersey-based Cuban Nationalist Movement, a militant Cuban exile group.

Townley was put in touch with Guillermo Novo Sampol and Jose Dionisio Saurez Esquivel (also a fugitive now and "told them the nature of his mission and requested their assistance," Cornick said.

A group meeting was set up on Sept. 12 with other leaders of the CNM at the Chateau Renaissance motel in North Bergen, N.J., at which Townley again discussed the Letelier murder plot and his need for assistance.

Attending the group meeting, the FBI agent said, were Novo, Suarez, Paz, Ross and a Newark physician, Dr. Juan B. Pulido. Pulido has not been charged in the investigation.

Within the next day or so, Paz called Townley and said the group had agreed to cooperate in the plot. (Cornick denied yesterday that any money was involved in the assassination plot and indicated the Cubans received nothing tangible for their cooperation with the DINA plot.)

Suarez and Novo got a plastic bag full of TNT in the Newark area and gave it to Townley, and he and Paz drove with the explosives to Washington in Paz's Volvo automobile on the evening of Sept. 15.

In Washington, Townley and Paz confirmed aspects of the surveillance conducted by Fernandez and bought some other parts used in the bomb, Cornick continued.

Suarez joined the two men in Washington on Saturday, Sept. 18, and they constructed the bomb. Around midnight that night or early Sunday morning Townley place the bomb atop the I-frame of Letelier's blue Chevelle so that its blast would be aimed directly at the driver, and Paz and Suarez assisted him.

After placing the bomb, Townley called Guillermo Novo in New Jersey and then flew back to the Newark area. In Newark on Sunday morning, he was met at the airport by Ross and told Ross about what had occured in Washington, Cornick said.

Ross drove Townley to Novo's apartment and Townley again repeated the details of what had occured in Washington, the agent testified.

The only person of the group who remained in Washington was Suarez, Cornick testified, and investigators believe that he activated the bomb from a car he was driving near Sheridan Circle NW. The bomb exploded about 100 feet from the Chilean ambassador's residence, but there is no indication that the Chilean embassy here was aware of or involved in the alleged murder plot.

Another FBI agent, Richard G. Sikoral, testified about the discovery of the "electric matches" in an apartment that had been rented in an assumed name by a person identified in court yesterday as Ross.

Sikoral said he had interviewed Ross last fall in connection with other unrelated bombings, and that at the time Ross said he was opening a business at a Union City, N.J., address.

Sikoral said he went to the Union City address about four months ago to question Ross again and found that an apartment there had been rented by a Carlos Garcia but was now vacant. The superintendent of the building identified a picture of Ross as Garcia, Sikoral added.

On March 8, Sikoral said, the building superintendent called FBI to report he had found what appeared to be bombing material in a cubbyhole in the apartment, which he was converting to his own private office.

The FBI agent described the material as "electric matches." Also found in the apartment were the three letters to Chilean officials and a small bottle of potassium permanganate, a "heat intensifying" chemical, the agent added under questioning by Assistant U.S. Attorney E. Lawerence Barcella Jr.

Cornick then took the stand again and said Townley had positively identified the electric matches found in the Union City apartment as being among the 10 he had brought into the U.S. from Chile. Townley said he had modified all of the electric matches in a special and easily identifiable manner, that one had been used in the Letelier blast and that he had left the other nine with Paz and Suarez.