WASHINGTON (AP) -- U.S. and Peruvian investigators are exploring whether a series of errors, rather than a single blunder, led to the mistaken downing of an American missionaries' plane over Peru, one of the investigators says.
The investigator also hinted at evidence the Peruvian military jet likely fired a required warning shot before downing the single-engine Cessna it suspected was carrying drugs.
One of the missionaries and her child were killed in the April 20 incident.
The investigator said he and his colleagues are moving toward finding that "there were several contributing factors that tragically conspired."
Among the issues being explored are:
--Whether there was a mix-up over radio frequencies used by the military and the missionaries' pilot.
--Whether flight plans were filed properly.
--Whether language translation problems aboard a CIA surveillance plane may have contributed to the tragedy.
And investigators are trying to determine if warning shots were fired, why the pilot of the missionaries' aircraft did not respond. The missionaries' group has said no warning shots were fired.
The investigator hinted that some evidence suggests warning shots were fired.
"It does appear that the Peruvians did follow their procedures and warning shots are an integral part of that," he said Wednesday, speaking on condition of anonymity. He declined to elaborate.
The missionaries' plane was being tracked by observers in a CIA surveillance plane, who initially identified it as a possible drug flight. U.S. officials have said that the CIA crew later realized it was likely an innocent flight, but couldn't stop the shooting.
The investigator said that a language problem aboard the surveillance plane "played a role" but that it has not been determined whether "it was a critical contributing factor."
The three Americans on the surveillance plane spoke little Spanish, though the CIA has said the Peruvian liaison officer aboard was proficient in English. Investigators want to know if poor communications between the Americans and the Peruvian, who was in contact with the military, might have played a part in the failure to properly and promptly identify the missionaries' plane.
The U.S. part of the investigative team, led by Assistant Secretary of State Rand Beers, returned Saturday from a weeklong trip to Peru and is continuing its work. U.S. members of the team also include representatives from the military, CIA and the multi-agency U.S. interdiction coordinator's office.
Investigators plan to interview the pilot of the downed aircraft, Kevin Donaldson, and another survivor, missionary Jim Bowers. His wife, Veronica, "Roni" Bowers, and her 7-month-old daughter, Charity, were killed.
The investigation is diplomatically sensitive because it could affect future drug interdiction policies of the two countries. Both U.S. and Peruvian officials have refused to discuss the shooting publicly.
The drug interdiction flights are suspended pending the outcome of the investigation.
Peru began shooting down suspected drug planes in the early 1990s to stop frequent flights carrying semi-processed cocaine to neighboring Colombia. The United States temporarily suspended sharing information about drug flights with Peru in 1994 until procedures could be set to prevent innocent planes from being fired upon.
Peruvian officials indicated those procedures were followed on April 20, the investigator said.
He said the Peruvian military tried to communicate with the missionary plane using civilian frequencies, but got no response. A key question still to be resolved is whether the missionary plane would have been using any of those frequencies at the time.
Another key issue concerns the flight plan. Peruvian officials said none had been filed. The missionaries' organization, New Cumberland, Pa.-based Association of Baptists for World Evangelism, contends a flight plan had been filed and has posted a copy on its Web site.
Peru's air force has reported shooting down 38 suspected drug planes since 1990. The policy is credited with the sharp drop in Peru's production of coca, the raw material of cocaine.
Critics of the policy, including some U.S. lawmakers, note that most of that coca production simply shifted into Colombia and question whether shooting at unarmed planes is legal under international law or moral.
On Wednesday, White House spokesman Ari Fleischer repeated the administration's view that drugs need to be fought both abroad and at home. "The problem is drugs grown in other nations ... enter the marketplace of America," he said.