The Department of Transportation's inspector general's office has severely rebuked National Highway Traffic Safety Administration (NHTSA) in a report that criticizes how the agency gathers and uses data for vehicle-defect investigations.

The report sums up a year-long investigation of NHTSA investigative practices that was launched at the behest of Sen. John McCain (R-AZ). McCain called for the review after Congressional hearings about the Firestone tread-separation debacle in 2000.

The DOT report found NHTSA often uses incomplete and “flawed” data to decide whether or not to open a vehicle-defect investigation. What's more, the report showed that in some cases NHTSA declined to open investigations despite reports of serious injuries-yet initiated them in defect cases that did not involve injuries.

In response, NHTSA has agreed to change its investigative practices, including by developing new methods to detect and analyze vehicle defects.