For more than a decade, the Japanese company Takata, one of the largest suppliers of airbags, denied that its products were defective even as motorists were killed by exploding airbags and automakers around the world recalled millions of cars equipped with its products.

But on Tuesday, in an about-face, Takata admitted that its airbags were defective and agreed to double the number of vehicles recalled in the United States, to nearly 34 million — or about one in seven of the more than 250 million vehicles on American roads — making it the largest automotive recall in American history. The airbags can explode violently when they deploy, sending shrapnel flying into a car’s passenger compartment. Six deaths and more than 100 injuries have been linked to the flaw.

“Up until now Takata has refused to acknowledge that their airbags are defective,” said Anthony Foxx, the transportation secretary. “That changes today.”

The announcement also indicated a shift for the National Highway Traffic Safety Administration, which for years had been criticized by lawmakers and safety advocates as being too lax on the industry it oversees. At one point, in 2009, the agency opened an investigation into Takata and its airbags, only to close it six months later, citing “insufficient evidence.”