Last Updated Oct 29, 2014 5:30 AM EDT
DETROIT - The U.S. government's auto safety agency, responding to criticism of its slow response to safety issues, told the manufacturer of millions of potentially faulty airbags to make replacement parts faster and do more testing to find the cause of the problem.
The National Highway Traffic Safety Administration sent letters Wednesday to Japanese airbag maker Takata and 10 automakers seeking information in a widening airbag recall that now covers almost 8 million U.S. vehicles.
The vehicles are equipped with Takata airbags that can potentially inflate with too much force, blowing apart metal canisters and sending shards flying at drivers and passengers. Safety advocates say four people have died due to the problem.
Tests by Takata have shown that prolonged exposure to high humidity can cause the inflators to malfunction. Some automakers have limited their recalls to a small number of high-humidity areas, but lawmakers and others are demanding that recalls be expanded nationwide.
Takata, the world's second-largest airbag maker with 22 percent of the market, has been plagued by problems for the past 13 years. For varying reasons, more than 12 million cars with its airbags have been recalled worldwide.
Honda (HMC) has been hit hardest in the latest round of recalls with about 5 million cars called back. Other affected automakers include Nissan (NSANY), Chrysler (FCAU), Ford (F), Mazda, Subaru, Mitsubishi, Toyota (TM), BMW and General Motors (GM). Lawmakers say 30 million cars with potentially faulty Takata airbags are driving on U.S. roads.
In the letter, NHTSA Deputy Administrator David Friedman told Takata that its inflators is are "creating an unacceptable risk of deaths and injuries by projecting metal fragments into vehicle occupants rather than properly inflating the attached air bag."
Letters to automakers urge them to speed up owner notifications and replacement-parts distribution. Friedman told Takata that those efforts won't work if it doesn't produce enough parts.
"Takata's production capacity is critically important," Friedman wrote.