In 2003, researchers at a federal agency proposed a long-term study of 10,000 drivers to assess the safety risk posed by cellphone use behind the wheel.

They sought the study based on evidence that such multitasking was a serious and growing threat on America’s roadways.

But such an ambitious study never happened. And the researchers’ agency, the National Highway Traffic Safety Administration, decided not to make public hundreds of pages of research and warnings about the use of phones by drivers  in part, officials say, because of concerns about angering Congress.

On Tuesday, the full body of research is being made public for the first time by two consumer advocacy groups, which filed a Freedom of Information Act lawsuit for the documents. The Center for Auto Safety and Public Citizen provided a copy to The New York Times, which is publishing the documents on its Web site.