U.S. safety regulators ordered Japanese automaker Honda Motor Co Ltd (7267.T) on Wednesday to provide documents and answer questions under oath about potentially defective air bags installed in millions of recalled U.S. vehicles.

The air bags were made by Japanese supplier Takata Corp (7312.T), which received a similar order last Thursday from the National Highway Traffic Safety Administration.

For the past 18 months, Takata has been besieged by chronic, widespread problems with defective inflators in its air bags, which can explode with excessive force and spray metal shards inside the vehicle. The issues have led to recalls at 10 automakers, but only Honda, Takata's biggest customer, has been sent a special order by the NHTSA.

The U.S. safety agency is seeking all internal Honda communications related to Takata air bag inflators, as well as any field reports, warranty claims and pre-suit legal claims, incidents and lawsuits related to the issue.

The agency also is asking Honda whether it sent any employees to visit Takata plants in the United States or Mexico, starting in 2000.

Honda was ordered to deliver the documents by Dec. 15.

NHTSA Chief Counsel Kevin Vincent issued an earlier special order to Honda on Monday as part of an investigation to determine whether the automaker failed to fully report accident-related deaths and injuries as required by U.S. law. The deadline for information pertaining to that order is Nov. 24.

"Honda has had regular communications with the NHTSA regarding the issues addressed in the special order, and we will continue to cooperate," the company said in an email statement.

Takata spokesman Alby Berman said the company "will continue to fully cooperate with the government investigation."

Since 2008, 10 global vehicle manufacturers that use Takata air bags have recalled more than 11 million cars in the United States and more than 17 million worldwide to replace inflators linked to at least four deaths and numerous serious injuries.

Honda has recalled nearly 7.6 million cars in the United States since 2008 because of the defective inflators, and more than 9.5 million cars globally.

One safety advocate said NHTSA's order was a must, given Honda's prominence in the various cases involving the potentially defective air bags.

"Honda had the first known Takata air bag inflator explosion in 2004 and has all 4 known deaths linked to Takata air bags," Clarence Ditlow, executive director of the Center for Auto Safety, said in an email.

Honda said later on Wednesday that it reported the incident to NHTSA as an injury claim in the second quarter of 2004 but did not connect it to the Takata issue until 2009.