By Richard Cowan, Alistair Bell, Patrick Rucker, and Ben Klayman

Five months after an exploding air bag killed a Virginia woman on Christmas Eve of 2009 the manufacturer offered the U.S. safety regulator additional documents, but the agency said it was unnecessary because it has just closed its investigation.

The victim died in a Honda car and at the time the probe focused on the carmaker and Takata Corp, its supplier, and the risk that air bags could rupture, shooting shrapnel into the car. The agency ruled that it did not find anything inappropriate in Honda’s actions.

The National Highway Traffic Safety Administration (NHTSA) told Reuters this week the additional documents “would not have added to the agency’s understanding of the issues involved in that particular investigation.”

Reuters was unable to review the documents. It is not clear what, if any, useful details the material offered by Takata in May 2010 contained or of it would have affected subsequent probes.

According to an e-mail exchange between Takata’s lawyer Kenneth Weinstein and the regulator’s investigator, the documents offered by the Japanese safety equipment maker looked at “other hypothetical causes” of air bag failures, but ones the manufacturer had eventually ruled out.

Still, one persistent critic, Clarence Ditlow, director of the Center for Auto Safety said it was alarming that the agency passed a chance to review the material.

“Once they close an investigation they just don’t like to look at anything new,” he said.

Executives of the agency, Takata and carmakers face a congressional hearing on Thursday about what has since become a major safety crisis linked to at least five deaths and massive recalls of over 16 million vehicles worldwide.

The hearing before the Senate Commerce Committee follows months of calls by several lawmakers and safety advocates for regulator’s more decisive federal government action.

Only this week the regulator told Takata and five carmakers most affected by the crisis to replace a patchwork of “safety campaigns” and regional recalls with a nationwide recall.

Democratic Senator Richard Blumenthal of Connecticut, another vocal critic said the agency has failed to identify safety defect trends early on.

“This office has been caught flat-footed repeatedly and egregiously,” Blumenthal told Reuters in an interview.