In November, top auto safety officials made an unusual request of Transportation Secretary Ray LaHood. After reviewing complaints about Toyota vehicles, the regulators said they believed the automaker was stalling their inquiries and wanted to go to Japan to stress just how serious their concerns had become.

Executives at Toyota “were dragging things out, and we’d had it,” a senior American transportation official said in recounting new details of the talks. “We were getting excuses that didn’t make sense anymore.”

Mr. LaHood approved the unprecedented trip, but it would be a month before his top safety aides met with Toyota executives, and another month before Toyota disclosed to Washington that it had found problems with some sticking accelerators. Not once in more than six years of reviews of Toyota’s problems did officials at the National Highway Traffic Safety Administration, which regulates automakers, use their power to subpoena Toyota’s records, even though they said they believed the automaker was withholding critical information.

Now, with the recalls of some eight million Toyota vehicles since late last year, including more than 400,000 models of the 2010 Prius and other hybrid models this week, the traffic safety agency promises to be scrutinized as much as Toyota itself. Members of Congress, independent experts on auto experts and others say they want to know why the agency did not act more aggressively in investigating Toyota’s problems.