The idea, Dr. Buckholtz said, was that the government’s National Institutes of Health “could serve as an honest broker between the pharmaceutical industry and academia.”

Soon, Dr. Richard J. Hodes, the director of the National Institute on Aging, was on the phone with Dr. Steven M. Paul, a former scientific director at the National Institute of Mental Health who had recently left to head central-nervous-system research at Eli Lilly. Dr. Paul offered to ask other drug companies to raise money.

It turned out to be relatively easy to get companies to agree, Dr. Paul said. It had become clear that the problem of finding good diagnostic tools was huge and complex. “We were better off working together than individually,” he said.

A critical aspect of the project was the Foundation for the National Institutes of Health, which was set up by Congress to raise private funds on behalf of the institutes. Dr. Paul was on its board.

In the end, the National Institute on Aging agreed to pay $41 million, other institutes contributed $2.4 million, and 20 companies and two nonprofit groups contributed an additional $27 million to get the project going and sustain it for the first six years. Late last year, the institute contributed an additional $24 million and the foundation was working on a renewal of the project for another five years that would involve federal and private contributions of the same magnitude as the initial ones.

At first, the collaboration struck many scientists as worrisome  they would be giving up ownership of data, and anyone could use it, publish papers, maybe even misinterpret it and publish information that was wrong.

But Alzheimer’s researchers and drug companies realized they had little choice.

“Companies were caught in a prisoner’s dilemma,” said Dr. Jason Karlawish, an Alzheimer’s researcher at the University of Pennsylvania. “They all wanted to move the field forward, but no one wanted to take the risks of doing it.”