Manila — Relations between Washington and Manila - including President Reagan's planned trip here in November and $900 million in American aid for the Philippines - hang in part on this question:
Who shot Benigno Aquino?
The family of Mr. Aquino, the only major political opponent to the regime of President Ferdinand Marcos, rejects the government's version of the shooting. Family members believe the assassin was one of three military officers who took Mr. Aquino off his plane at Manila airport Aug. 21.
Full knowledge of the facts surrounding the assassination awaits further evidence and an official probe by the government of Ferdinand Marcos.
(A leading US critic of the Marcos regime, Rep. Stephen Solarz (D), has called for an ''impartial'' investigation of the killing and warned of trouble for the $900 million aid package pending in Congress that is tied to a new military base agreement with the Philippines. And although Mr. Reagan still plans to visit the Philippines, the administration is watching whether a ''thorough and objective'' investigation of the killing is conducted.)
Serious doubts already have been raised about the government's version of how Mr. Aquino was shot. This reporter, for instance, arranged to gain access to the official autopsy report on Mr. Aquino. It provides circumstantial evidence that the man alleged by the government to have fired the shot could not have been able to position himself in such a way as to account for the path of the bullet that slayed Aquino.
Also putting the government under suspicion is a claim by members of the Aquino family that they were denied access to the ramp leading to the China Airways plane carrying Aquino. This occured even though they had earlier received a verbal promise by officials that they would have access.
On Monday evening, Mr. Marcos appeared on television, saying Aquino had probably been killed by a single assassin working for the communist underground.
''We don't believe any word he said last night,'' said Teresa Aquino Oreta, a sister of Aquino, speaking on behalf of the family.
''It was obvious that Ninoy (Mr. Aquino's nickname) was boxed in by the Avsecom (Aviation Security Command) guards at the airport,'' said Ms. Oreta. The family did not feel the alleged assassin could get close enough to kill Aquino.
''The projection of the bullet shows that it was fired by someone taller than Ninoy, or someone in a higher place,'' said Mr. Oreta. ''As he came down the airplane steps, there was one Avsecom guard on his right, one on his left. Where was the third?''
Ms. Oreta believes Aquino was shot at or near the foot of the plane steps. ''He was shot from behind because the force of the bullet threw him forward, face down,'' she said.
Marcos claimed the bullet had been fired from close range, perhaps 16 to 18 inches. Earlier on Monday, Salvador Laurel, an opposition leader, said that an official carrying out the autopsy had initially estimated the shot had been fired from 2 to 3 inches. Mr. Laurel said the official had opted for the 18-inch figure to be on the safe side.
The autopsy was carried out by the medical section of the Justice Ministry's National Bureau of Investigation. Military doctors and medical representatives of Aquino's family were also present. The autopsy confirms that Aquino was hit with a single bullet that entered behind the left ear and was directed ''forward , downward, and medially.'' The bullet exited at the chin.
An assassin slightly shorter than Mr. Aquino would have had difficulty in shooting at that angle. Aquino's sister and other members of the family said Aquino was 5 feet, 9 inches tall. Newspapers in Manila quote Manila Police Commander Prospero Olivas as saying that Aquino's alleged assassin was 5 feet, 7 inches tall.
Ms. Oreta said that to the best of her knowledge, Aquino did not receive any direct death threats while in the US. And, she added, the family did not receive any after Aquino announced his plans to return home in June.
''The only death threats,'' Ms. Oreta continued, ''were the ones orchestrated by the newspapers here - the press releases from the administration saying they did not want Ninoy to come home because of the risks of assassination.
''But, of course, the only (specific) death threat facing Ninoy was the death sentence facing him.'' Aquino was sentenced to death by a military tribunal in 1977.
Ms. Oreta repeated the claim that relatives and political associates of Aquino had been given verbal permission last week to meet him at the ramp from the plane to the passenger terminal. The permission, Ms. Oreta said, had been given verbally to Laurel by Manila international airport manager Luis Tabuena last Friday. When family members and supporters arrived at the airport on Sunday , however, they were told it was impossible to approach the plane.
(Neither Mr. Tabuena nor the chief of the Aviation Security Command, Gen. Luther Custodio, have been available for comment on the claim. Repeated attempts to contact General Custodio by phone proved impossible. A person answering the phone in his office said the general was in another office elsewhere in Manila and that she did not know the other number. She then hung up.)
While the family was not able to get to the plane, the killer was, Ms. Oreta said. ''He had access to the maximum security area and to the Aviation Security Command,'' she said.
The number of people coming to see Aquino's body has increased. Early Tuesday , about 2,000 people waited in line. ''We're clocking them at 35 a minute,'' said a family member, who claimed 32,000 people had passed through the house Monday. Before they enter, they are handed a small piece of black cloth to put on their clothing; as they leave, family members shake each person'shand and thank them. The family does this energetically, determinedly, like political activists developing their constituency.
Meanwhile, the government announced that it had disarmed 14 Avsecom soldiers who had been on duty in the area of the killing Sunday. They would be subjected to routine paraffin and ballistic tests, Metropolitan Police Chief General Olivas said Tuesday, to try to find out who had been using which weapons.
Photographs taken immediately after the killing, however, show that a number of security men were firing their weapons or preparing to do so. And sources said that paraffin tests - to determine whether a person has fired a gun - are normally effective only up to 24 hours after the incident.