By any measure, former Washington Post executive editor Len Downie epitomized success in the traditional, subscription-and-advertising model of newspaper journalism: With a staff that once topped 900 and an annual budget of $100 million, his newsroom hauled in 25 Pulitzer Prizes over 17 years and wielded influence from Congress to the darkest recesses of the nation’s capital.

Since stepping down from the Post’s top newsroom job at age 66, Downie has taken on a professorship at Arizona State University. But behind the scenes, he also is lending his experience to help shape the practices and prospects for the burgeoning nonprofit sector in journalism.

Why? Simple: Downie says the for-profit model alone no longer can support the kinds of investigative, explanatory, and accountability journalism that society needs. As the for-profit sector shrinks, journalists and interested readers must explore new ways to underwrite their work.

“There are going to have to be many different kinds of economic models,” Downie said in an interview at the Post’s offices. “The future is a much more diverse ecosystem.”

Downie has made himself an expert on the nonprofit model, and wrote about its possibilities in his recent report, “The Reconstruction of American Journalism,” with Michael Schudson.

Less known, perhaps, is that Downie casts a wide net as within the nonprofit sector of journalism. He’s a board member at the Center for Investigative Reporting, which recently launched California Watch to cover money and politics at the state level. He also chairs the journalism advisory committee at Kaiser Health News, which has provided niche explanatory reporting to leading newspapers, including the Post. And he’s also on the board of Investigative Reporters and Editors, which has incorporated panels on the nonprofit model into its conferences. (I should note that I am a part-time editor for the Washington Post News Service.)

Looking across the sector, Downie sees great potential — and some big, unanswered questions.

On the upside, nonprofits are helping journalism move toward a more collaborative model, Downie said. In the old days, newspapers resisted ideas and assistance from outside. But in the new news ecosystem, collaboration is a way of life. “All of our ideas have been changed about that,” he said.

Also a plus: Big foundations and the public at large are warming to the idea that news organizations are deserving of their support, just like the symphony or any other nonprofit that contributes to society’s cultural assets. “There’s a question of whether there’s enough public realization,” Downie said. “I think we’re heading to that direction. Awareness is growing steadily.”

But a lot of questions still must be sorted out, Downie said.

High on the list, he said, is the most basic of all: Where will the money come from? Like other nonprofits, nonprofit news organizations will have to find the right mix of foundation money, grassroots support, advertising, and perhaps additional government support, he said.

That leads to the other big question of sustainability: It’s not clear that all the nonprofits that have launched in recent years will survive. “How many will succeed and for how long?” Downie wondered. A related question: How will the collaborative model will settle out, and where nonprofits will find productive niches?

Downie said he also has been watching nonprofits wrestle with the issue of credibility — how to achieve it and how to keep it.

The answer begins with editorial independence and transparency about financial supporters, Downie said. But when it comes to painting a bright line between journalism and ideology, advocacy or spin, there are no magic formulas to assure readers — just the experience of trial and error.

“It’s one of these things that’s proven by its exceptions,” Downie said. “When there’s an exception, it’s a scandal.”