In his reporting, Fahrenthold had learned that Trump’s charity paid for portraits of the businessman, potentially violating laws regulating how charities operate. One key question that would help determine the legality of the move was the use to which the portraits had subsequently been put. The location of one was already known, but not of a second picture that Fahrenthold’s reporting had turned up.

Other reporters might have quietly embarked on a search, racing to be first to the scoop. Fahrenthold, instead, asked his Twitter followers for help. An internet—and real-world—treasure hunt ensued. And in the space of just a few hours, with the help of an Atlanta woman, Allison Aguilar, and a Florida Univision journalist, Enrique Acevedo, the location became known. Here’s the sequence, as Fahrenthold described it in an email:

1) 10:34 a.m., 9/20/2016. I posted this tweet, revealing the new portrait and linking to a story asking where it was:

ALSO...There is another.@realDonaldTrump bought 2nd portrait of himself w/ charity money. This time, for $10K. https://t.co/JnBuBLu3fS pic.twitter.com/KnAFMzIzuZ — David Fahrenthold (@Fahrenthold) September 20, 2016

2) Less than an hour later, at 11:04 a.m., Allison Aguilar (@alkaguilar) tweeted at me that she’d found it:

@Fahrenthold You don't follow me, so can't DM, here is the $10K one at Doral National in Miami. ref Tripadvisor :https://t.co/uomyGGduuN — ALKA (@alkaguilar) September 20, 2016

3) I didn't see this, in the avalanche of twitter traffic, so I didn’t see that Aguilar had found it until that night, when she sent me an email.

4) 7:11 p.m., 9/20/2016 When I did see her message, I tweeted out that Aguilar had found it in a photo from trump's golf course at Doral.

Thanks to awesome, amazing WaPo reader @alkaguilar for spotting this. https://t.co/gXCl0l8WvC — David Fahrenthold (@Fahrenthold) September 20, 2016

5) Then, at 12:30 a.m. that night, Enrique Acevedo Tweeted at me that he’d found the portrait in real life:

Hey @Fahrenthold just checked and the portrait is still hanging at the Champions Lounge. How much did you say it cost the Trump Foundation? pic.twitter.com/hGAun6KgCO — Enrique Acevedo (@Enrique_Acevedo) September 21, 2016

“It was an amazing day,” he told me. “I never would have found it myself.”

One of Fahrenthold’s most clever transparency techniques combined low-tech with digital media: He made a list, with a pen and paper, of “more than 400 charities with some ties to the GOP nominee in an effort to find proof of the millions he has said he donated to them,” and posted results as he got them. (“We’ve been mostly unsuccessful,” he wrote dryly in the Post.)

Trump and his supporters are known for trolling and attacking their critics, including people in the news media. Perhaps surprisingly, Fahrenthold says he didn’t get much abuse. “Trump called me a ‘nasty guy’ on the phone, and some of his surrogates called me ‘obsessed’ and biased on TV,” he said. “But I heard almost nothing from the Trump campaign, and very little from Trump supporters (at least compared to the reports I heard from others).”