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.

In interviews, the officials who withheld the research offered their fullest explanation to date.

The former head of the highway safety agency said he was urged to withhold the research to avoid antagonizing members of Congress who had warned the agency to stick to its mission of gathering safety data but not to lobby states.

Critics say that rationale and the failure of the Transportation Department, which oversees the highway agency, to more vigorously pursue distracted driving has cost lives and allowed to blossom a culture of behind-the-wheel multitasking.

“We’re looking at a problem that could be as bad as drunk driving, and the government has covered it up,” said Clarence Ditlow, director of the Center for Auto Safety.

The group petitioned for the information after The Los Angeles Times wrote about the research last year. Mother Jones later published additional details.

The highway safety researchers estimated that cellphone use by drivers caused around 955 fatalities and 240,000 accidents over all in 2002.

The researchers also shelved a draft letter they had prepared for Transportation Secretary Norman Y. Mineta to send, warning states that hands-free laws might not solve the problem.

That letter said that hands-free headsets did not eliminate the serious accident risk. The reason: a cellphone conversation itself, not just holding the phone, takes drivers’ focus off the road, studies showed.

The research mirrors other studies about the dangers of multitasking behind the wheel. Research shows that motorists talking on a phone are four times as likely to crash as other drivers, and are as likely to cause an accident as someone with a .08 blood alcohol content.

The three-person research team based the fatality and accident estimates on studies that quantified the risks of distracted driving, and an assumption that 6 percent of drivers were talking on the phone at a given time. That figure is roughly half what the Transportation Department assumes to be the case now.

More precise data does not exist because most police forces have not collected long-term data connecting cellphones to accidents. That is why the researchers called for the broader study with 10,000 or more drivers.

“We nevertheless have concluded that the use of cellphones while driving has contributed to an increasing number of crashes, injuries and fatalities,” according to a “talking points” memo the researchers compiled in July 2003.

It added: “We therefore recommend that the drivers not use wireless communication devices, including text messaging systems, when driving, except in an emergency.”

Dr. Jeffrey Runge, then the head of the highway safety agency, said he grudgingly decided not to publish the Mineta letter and policy recommendation because of larger political considerations.

At the time, Congress had warned the agency not to use its research to lobby states. Dr. Runge said transit officials told him he could jeopardize billions of dollars of its financing if Congress perceived the agency had crossed the line into lobbying.

The fate of the research was discussed during a high-level meeting at the transportation secretary’s office. The meeting included Dr. Runge, several staff members with the highway safety agency and John Flaherty, Mr. Mineta’s chief of staff.

Mr. Flaherty recalls that the group decided not to publish the research because the data was too inconclusive.

He recalled that Dr. Runge “indicated that the data was incomplete and there was going to be more research coming.”

He recalled summing up his position as, the agency “should make a decision as to whether they wanted to wait for more data.”

But Dr. Runge recalled feeling that the issue was dire and needed public attention. “I really wanted to send a letter to governors telling them not to give a pass to hands-free laws,” said Dr. Runge, whose staff spent months preparing a binder of materials for their presentation.

His broader goal, he said, was to educate people about the dangers of distracted driving. “Based on the research, there was a possibility of this becoming a really big problem,” he said.

But “my advisers upstairs said we should not poke a finger in the eye of the appropriations committee,” he recalled.

He said Mr. Flaherty asked him, “Do we have enough evidence right now to not create enemies among all the stakeholders?”

Those stakeholders, Dr. Runge said, were the House Appropriations Committee and groups that might influence it, notably voters who multitask while driving and, to a much smaller degree, the cellphone industry.

Mr. Mineta, who left as transportation secretary in 2006, said he was unaware of the meeting.

“I don’t think it ever got to my desk,” he said of the research. Mr. Ditlow, from the Center for Auto Safety, said the officials’ explanations for withholding the research raised concerns. He said the research did not constitute lobbying of states.

And he said it was consistent with the highway safety agency’s research in other areas, like seat belts.

Mr. Ditlow said that putting fears of the House panel ahead of public safety was an abdication of the agency’s responsibility.

“No public health and safety agency should allow its research to be suppressed for political reasons,” he said. Doing so “will cause deaths and injuries on the highways.”

State Senator Joe Simitian of California, who tried from 2001 to 2005 to pass a hands-free cellphone law over objections of the cellphone industry, said the unpublished research would have helped him convince his colleagues that cellphones cause serious — deadly — distraction.

“Years went by when lives could have been saved,” said Mr. Simitian, who in 2006 finally pushed through a hands-free law that took effect last year.

The highway safety agency, rather than commissioning a study with 10,000 drivers, handled one involving 100 cars. That study, done with the Virginia Tech Transportation Institute, placed cameras inside cars to monitor drivers for more than a year.

It found that drivers using a hand-held device were at 1.3 times greater risk of a crash or near crash, and at three times the risk when dialing compared with other drivers.

Not all the research went unpublished. The safety agency put on its Web site an annotated bibliography of more than 150 scientific articles that showed how a cellphone conversation while driving taxes the brain’s processing power, impairing reaction time. But the bibliography included only a list of the articles, not the one-page summaries of each one written by the researchers.

Chris Monk, who researched the bibliography for 18 months, said the exclusion of the summaries took the teeth out of the findings.

“It became almost laughable,” Mr. Monk said. “What they wound up finally publishing was a stripped-out summary.”

Mr. Monk and Mike Goodman, a division head at the safety agency who led the research project, theorize that the agency might have felt pressure from the cellphone industry. Mr. Goodman said the industry frequently checked in with him about the project and his progress. (He said the industry knew about the research because he had worked with it to gather some data).

But he could offer no proof of the industry’s influence. Mr. Flaherty said he was not contacted or influenced by the industry.

The agency’s current policy is that people should not use cellphones while driving. Rae Tyson, a spokesman for the agency, said it did not, and would not, publish the researchers’ fatality estimates because they were not definitive enough.

He said the other research was compiled as background material for the agency, not for the public.

“There is no report to publish,” he said.