The safety agency has repeatedly suggested that it failed to act over the years because of a lack of a critical mass of evidence that suggested a problem beyond isolated incidents.

In a statement emailed to The Times, a spokesman noted that over the last seven years, the agency’s investigations in other cases have resulted in 929 recalls of more than 55 million vehicles. The agency “uses a number of tools and techniques to gather and analyze data and look for trends that warrant a vehicle safety investigation and possibly a recall,” the statement said. The agency said 260 complaints amounts to about .018 percent of the vehicles under recall.

In a telephone interview, Frank Borris, the director of the agency’s office of defects investigation, said that two years ago, the agency began using IBM software to help look for patterns. Judgments are made by “really well-seasoned automotive engineers who leverage a lot of technology and lean on past precedent about when to open, when to close, and when to push for a recall,” Mr. Borris said. “It’s no magic formula.”

The agency’s chief counsel, Kevin Vincent, said that to warrant an investigation, the case must pass a legal test of “unreasonable risk to safety.”

“That term ‘reasonable’ is a legal term, which is very elastic and means a lot of different things in a lot of different contexts,” he said. “Each case is a different fact pattern.”

The recall has thrown G.M. into turmoil just as it was emerging from the shadow of bankruptcy under the leadership of a new chief executive, Mary T. Barra, and as the safety agency stepped up pressure on the company in late February by opening an investigation into the “timeliness” of G.M.’s “defect determination.” Last week the agency sent the company 107 questions, demanding it explain why it had waited so long to recall the vehicles. G.M. officials had known for a decade that the switches could fail.