Two years ago, when the Justice Department forced New Jersey officials to adopt new policies to discourage racial profiling by state troopers, it also told the state to study the driving habits of black and white motorists on the New Jersey Turnpike.

The task was complicated, but the reason for it was simple: numerous studies have shown that police officers in New Jersey and elsewhere stop black and Hispanic drivers for speeding more often than they stop whites. What is less certain is why -- how much of that disparity is because of racial profiling and how much, if any, is attributable to differences in driving behavior, which have never been adequately documented.

But rather than clarifying the issue, the study created its own muddle. Justice Department officials say they have such serious questions about the methods used to gather the data that they have asked New Jersey's attorney general not to release the findings. It is not clear whether they will be made public.

The study involved photographing tens of thousands of drivers on the turnpike last spring while clocking speed with a radar gun. It found that black drivers sped much more than other drivers, according to three people who have reviewed the unreleased report. The racial gap was far wider than officials had expected and, in the politically charged controversies over profiling, the data could be used by defenders of the state police to argue that one reason black drivers are stopped more often than whites is that they are more likely to speed.