At the beginning of the year, we asked ProPublica Illinois readers what they wanted to know about how we do our work. Thoughtful, challenging questions have been rolling in ever since, and we’ve been answering them in an occasional series of columns. In this dispatch, ProPublica Illinois reporter Jason Grotto answers a question about vetting anonymous sources.

How do you verify that an anonymous source is being truthful? I have worried that all these leaks, from White House sources, are an attempt to discredit our free press. The truth really does matter, so how do you check the integrity of your sources? — Paula Wyatt

In the movie “All the President’s Men,” a young Bob Woodward repeatedly goes to a dimly lit parking garage to meet with a man whose face was shrouded by shadows as he dispenses bits of information. That information fuels the Washington Post’s reporting on Watergate and, ultimately, helps lead to the resignation of President Richard Nixon.

The source was known as “Deep Throat,” and he became perhaps the most famous anonymous source in modern journalism history. But he wasn’t anonymous to Woodward, who near the beginning of the movie calls him from a pay phone hoping he’ll provide information about the break-in at the Democratic National Committee headquarters, located in the Watergate complex.

In mid-2005, Mark Felt — a high-ranking FBI official at the time Watergate was unfolding — was identified in a Vanity Fair magazine story as “Deep Throat.” The Post confirmed the identification.

More often than not, the journalist and anonymous source know each other, as Woodward and Felt did. These relationships are built on trust, the kind that develops over time as information is provided or, in some cases, exchanged. Journalists then try to verify the information through documents or other sources.

I have never quoted an unnamed source in a story. But I’ve had sources who are anonymous to readers. They are a key part of the work we do but must be managed carefully to protect them and guard the integrity of our work. Wrong information from a source — especially an unnamed source — can undermine a story and damage a reporter’s career and a news organization’s credibility.

In many instances, anonymous sources never appear in stories. They figure in the background of stories, providing reporting road maps or insights into information already gathered by a reporter.

When I was working on a series about Cook County’s unfair property tax assessments, I had several sources who never appeared in the stories. Instead, they gut-checked information or offered perspectives from inside the world of tax assessments. In some cases, they gave me tips about where I should look for information, not unlike the advice Woodward received: “Follow the money.”

Make no mistake: These relationships are tricky. And, if handled badly, they can lead to questionable conduct on both sides. That’s why it’s crucial for reporters to disclose the identity of their anonymous sources to editors.

It’s important to understand a source’s motivation for speaking and determine his or her agenda. I always tell sources I will verify everything they tell me. As long as I’m aware of their agenda, I can balance the information they give me with other relevant information. Or challenge their information with other facts to build a story that includes multiple points of view.

Some news outlets prohibit quoting anonymous sources or allow it only when naming them will jeopardize their work or safety. Anonymous sources are usually identified in a story with some additional description — “sources familiar with the matter” or “speaking on condition of anonymity.”

At ProPublica, our code of ethics says that we try to identify all sources of information and grant anonymity to sources “only when they insist upon it and when they provide vital information” as well as “when there is no other way to obtain that information” and when we know the source is “knowledgeable and reliable.”

Editors play a crucial role in the decision to use an anonymous source. Our code of ethics says editors “have an obligation to know the identity of unnamed sources” so they can “assess the appropriateness of using their information.”

One of journalists’ guiding principles when dealing with anonymous sources is confidentiality. But that protection is limited to doing what we can to keep them anonymous. New York Times reporter James Risen waged a seven-year legal battle over whether he would have to reveal an anonymous source in court testimony. In the end, he didn’t have to. And, of course, there’s Woodward and his reporting partner Carl Bernstein’s decades long silence on the identity of “Deep Throat.”

As with all other information we gather, we try to verify that information using public documents, on-the-record sources and reaction from those affected by the story, especially if it is detrimental to a person or an institution.

Even if we don’t have to meet them in a parking garage.