WASHINGTON — Lawmakers expressed frustration Tuesday over the progress of a recall of millions of defective air bags, pointing out that it’s still unclear what’s causing the air bags to rupture, which vehicles need repairs and whether replacement air bags are truly safe.

“Every morning I feel like I am playing headline roulette, waiting for another rupture, another injury, another death,” Rep. Michael Burgess, R-Texas, said at a subcommittee hearing to question federal regulators and officials from Takata, the Japanese company that makes the air bags.

Takata agreed last month to expand its year-old recall to 33.8 million air bags defective under pressure from the National Highway Traffic Safety Administration, which is now overseeing the recall and coordinating repairs. Faulty inflators inside the air bags can explode with too much force and spew shrapnel into the passenger compartment. The problem is responsible for six deaths and more than 100 injuries worldwide.

When Burgess asked NHTSA chief Mark Rosekind if Takata’s replacement air bags are safe, Rosekind acknowledged that “a definitive root cause has not been identified,” and the government may never actually know the cause. He said dealers should tell owners whether they’re getting an interim fix or a permanent fix when they bring in their cars to get replacement air bags.

Rosekind also said drivers should check NHTSA’s website, www.safercar.gov, weekly to see if their car is being recalled.

The agency is still getting lists of affected vehicles from automakers. He said it will be fall before the agency has a timeline for all the repairs to be completed.