ATHENS, Greece (AP) _ The pilot of a TWA jetliner that was ripped open by a bomb while approaching Athens Airport said today the explosive probably was placed under a passenger seat.

''The passengers' legs were damaged in the lower part of the leg. The explosion was in the cabin,'' Capt. Richard Peterson told reporters.

Greek officials had said immediately after the bombing Wednesday that the explosive was in the cargo hold, under the passenger cabin. However, TWA President Richard D. Pearson said in New York that the explosion occurred on the floor of the cabin in seat row 10 or 11.

The pilot agreed, saying, ''The explosion was above the floor. It looked like it must have been beneath a seat.''

Peterson, 54, from Sarasota, Fla., said debris that fell from the Boeing 727 as it flew over Argos, 60 miles south of Athens, probably would reveal how the explosion occurred.

''They'll probably find something in Argos, where they found the bodies, that would indicate whether it was a bag or satchel or something with powder marks on it,'' he said.

The bodies of a Colombian-born U.S. citizen and three members of a Greek- American family - a mother, daughter and baby granddaughter - were found Wednesday near Argos. Part of a passenger seat and other debris also were recovered.

Peterson spoke to reporters at Athens Airport while waiting to give a news conference organized by TWA. But he was led away abrubtly by company officials who said the crew members had to be questioned by U.S. Federal Aviation Administration and FBI investigators. The news conference was canceled.

TWA Flight 840 took off from Rome with 122 people on board and headed for Athens and Cairo, Egypt. The bomb exploded about two hours later as the plane neared Athens and tore a hole in the cabin wall in front of the right wing.

''The whole aircraft was filled with debris,'' Peterson said. ''I told Athens Airport we were on an emergency and we were coming directly to the field.''

The plane landed safely 10 minutes after the explosion, Greek officials said.