Japanese auto parts maker Takata Corp. manipulated air bag test data that it supplied to Honda Motor Co., sources said, citing a third-party investigation.

The investigators examined the data after Takata, which has faced massive global recalls over defective air bags, acknowledged that parts of its test data for gas generators were inappropriate, as pointed out by Honda last November.

A Takata official refused to comment on the details of the data manipulation investigation, telling Kyodo News, "We have already implemented thorough measures to prevent a recurrence and will continue to work to improve the situation."

It is estimated that costs related to the recalls, which will be shouldered by Takata and some automakers, will reach ¥1 trillion (about $9.5 billion). The latest revelations may affect negotiations over how much each party pays.

Globally, at least 15 deaths have been linked to defective Takata inflators, which spray shrapnel after deploying.

Bloomberg News reported the data manipulation on Monday, quoting a former official who was once part of the third-party panel looking into Takata's air bag tests results.

"We have found examples of what I would call 'selective editing,' where they have left out results, not because they were bad results, but because the results that remained were better," Brian O'Neill, a former U.S. Insurance Institute for Highway Safety president, told Bloomberg.