This woman really likes Donald Trump. (Jabin Botsford/The Washington Post)

On Tuesday, WaPo’s Dave Fahrenthold got to the bottom of one of the big mysteries of Donald Trump’s presidential campaign: Did he really raise and donate $6 million — including $1 million of his own money — to veterans’ organizations as he had boasted for months?  I was fascinated by how Fahrenthold harnessed the power of social media to finally pin down Trump and get some answers. I reached out to Dave to pick his brain about why the story initially interested him and how and when he decided to take his reporting to Twitter. Our conversation, conducted via email and edited only for grammar, is below.

Explain your thought process when you decided to take on the Trump veterans donation claim. What drew you to it?

I’d seen Trump give away oversized checks to vets’ groups in Iowa — and then stop giving them away in New Hampshire, with much of the money still unaccounted for. I decided to see if I could find out what happened to the rest of it. I liked this story idea because it was so concrete. There was no need for speculation: “Could Trump really build the wall? Experts say …”). There was a specific claim — Trump had raised $6 million, and he was giving it away. And there were people outside Trump’s campaign (the vets’ charities) who could say whether that claim was actually coming true.

I started this as a side project in February, making a few calls to the charities Trump had listed as recipients. At that point, I figured it would most likely be a wild goose chase. I’d find out that the charities had all already received big donations from Trump, and that would be that. Because who stiffs military veterans in the middle of a presidential campaign? 

But then, the first ones I called said they hadn’t actually received anything yet from Trump. That got me started on what’s now been a few months of reporting. 

You used Twitter to report the story out. Why? And what was the result?

By this week, I thought I had a good handle on one part of this story: what became of the donations that other donors gave to Trump’s effort. We knew that at least $4.5 million had been raised, and $3.1 million had been given away so far. The piece that was missing was the $1 million that Trump said he’d given himself. His campaign manager, Corey Lewandowski, told me Trump had given the money away. But to who? Lewandowski wouldn’t say. 

I had doubts: Would Donald Trump, of all people, really give away $1 million in secret?

But that left me trying to prove a negative. I could try to call every veterans’ charity in the U.S. and ask them all individually if they’d received money from Trump. But even if I reached 1,000 charities, what if Trump had given his money to number 1,001?

I decided to make my search public, using Twitter to reach out to prominent veterans’ groups, vets advocates and news sites aimed at veterans and active-duty military. I asked whether any of them had received even $1 from Trump’s supposed million-dollar gift. I used Trump’s Twitter handle in my queries so they would show up in Trump’s own mentions. I wanted Trump to feel scrutiny on his preferred social-media platform in the hope that he would reach out to answer the question himself. Apparently, he noticed. That night, Trump sent several tweets attacking the media for its coverage of his fundraiser. And — at some point that evening — he actually made a call and gave his $1 million gift, almost four months late.

When did you realize that Trump hadn’t actually made the donation he said he made?

Not until Trump called me on Tuesday afternoon to say he’d just given the whole million at once. Until then, I hadn’t found proof of any gift of personal cash from Trump to a vets’ group. But of course, I also hadn’t been able to prove the negative: that such a gift definitely didn’t exist.

How did you get Trump on the phone? At what point in the story did it happen?

We had put in a request to talk with Trump on Tuesday, after he’d sent out those tweets and an Instagram message criticizing our reporting. But to be honest, I didn’t think he would call: In the past, my questions about the vets fundraising have been answered by Trump’s staffers, not the candidate himself.

But then, about 5 p.m., he did call from his plane. We had to scrap our old plan for that day’s story, which had been mainly focused on Trump’s attacks on the media. After I got off the phone with Trump, I called the foundation he’d given to and got confirmation that the $1 million had truly been promised to them. Then we quickly wrote up a story for the Web.

This was a story you used social media to execute. What’s been the response on Twitter and the like once the story landed?

I was really grateful that many other reporters – including many who don’t even cover politics – retweeted my queries seeking answers about Trump’s $1 million on Monday. I’m not much of a social-media expert, and my Twitter following is pretty measly. I got a lot of help from Post colleagues with bigger followings, of course. But I also got a huge boost from people like Matt Pearce at the L.A. Times and Sopan Deb at CBS (whom I offered to buy dinner if he could find me somebody who got Trump’s money). They amplified my questions, helped me reach a far larger audience than I could have on my own. It was wonderful to get that kind of vital help from people who are theoretically my competitors.

Since the story landed, the reception has been incredibly gracious. And I am glad that I don’t have to buy Sopan dinner after all.