His concerns about the effectiveness of certain imaging devices for detecting breast and colon cancer are shared by some medical experts. But potential allies were driven off by his abrasive style. “He got annoying, he got obstructive in nature,” said Dr. Carl D’Orsi, a mammography expert at Emory University who worked on an F.D.A. review panel and also served as a consultant for a manufacturer. Fellow employees went further, complaining to the consultant that Dr. Smith was “disruptive,” “adversarial” and “confrontational.”

Staff members for three Congressional committees that often criticize the F.D.A. reviewed Dr. Smith’s complaints and chose not to pursue them. The inspector general of the Department of Health and Human Services examined his claim that the agency was violating the law in reviewing medical devices and concluded that he was wrong.

Dr. Smith maintains that many imaging devices do not work as advertised by their manufacturers, produce many false positives and subject patients to needless rounds of potentially harmful radiological testing. Moreover, he says they waste enormous amounts of federal Medicare aid. Dr. Smith said that his sole goal in making his complaints was to protect Americans from harmful medical devices and that money was not a motive.

His concerns are now getting another look. The Office of Special Counsel, which investigates whistle-blower grievances, found in a confidential review this spring that Dr. Smith’s allegations raised a “substantial likelihood” of serious problems and required a full review. That has triggered an investigation by Kathleen Sebelius, the secretary of health and human services.

A graduate of Johns Hopkins University and Yale Medical School, Dr. Smith was a respected radiologist who set new standards for diagnosing kidney stones with groundbreaking research in the mid-1990s. Colleagues described him as both brilliant and bombastic, and he clashed with supervisors in his jobs on the medical faculties at Yale and Cornell.

He charged in lawsuits that both universities fraudulently billed Medicare for improper radiological services. The fraud claims were ultimately thrown out, but Dr. Smith won settlements believed to total several hundred thousand dollars over retaliation he said he endured for complaining.

Within two years of his joining the F.D.A. as a radiologist in 2006, relations began to turn acidic there as well, as he challenged the approval of more than a dozen imaging devices.