The U.S. military has taken precautions to keep its communications secure in the event that China learned secrets from the Navy surveillance plane detained on Hainan, a Defense Department spokesman said on Thursday.
"I will say that we have taken what we consider to be prudent measures to minimize whatever compromise there may have been. But I can't provide you more details," Rear Adm. Craig Quigley said.
The EP-3 surveillance plane made an emergency landing on the island on Sunday after a collision over the South China Sea with a Chinese fighter jet, which crashed.
U.S. officials say Chinese authorities have examined the plane, which carries highly secret equipment for eavesdropping and other sensitive material.
One concern was that China might be able to discover the U.S. military's internal system for secret communications.
But one U.S. official, on condition of anonymity, said that any compromise of that system would be short-lived because encryption codes changed frequently.
U.S. authorities do not have a complete idea yet of how much of the secret surveillance equipment and materials on the plane was destroyed before it landed because only one visit has been allowed with the 24-member crew detained on Hainan.
"You have procedures that the crews of aircraft, ships and submarines are trained to do emergency destruction of their prioritized, classified sorts of equipment," Quigley said.
"How much of that they got through - again, we need to talk to them directly to ascertain that," he said.
Defense officials, on condition of anonymity, have said that the crew of the plane destroyed all sensitive reconnaissance data on board.
Asked how that determination had been made, Quigley said, "You would expect a well-trained crew to carry out their training and the emergency destruct procedures as best as they could, given the limited time frame available to them."
The plane incident has blown up into major diplomatic friction between the United States and China. The United States is seeking the return of the crew and plane, while China is seeking an apology.