It was Spain’s worst rail disaster in 70 years. An express train careened off the tracks in a jumble of flying steel, killing 79 people. In chaos that followed one key question emerged almost instantly: Was the train driver speeding? Initial but unsourced reports indicated he was. Video of the crash from a security camera seemed to show this. But as in the immediate aftermath of most disasters, precise, reliable information was very hard to come by.

This combo image taken from security camera video shows clockwise from top left a train derailing in Santiago de Compostela, Spain, on Wednesday July 24, 2013. Spanish investigators tried to determine Thursday why a passenger train jumped the tracks and sent eight cars crashing into each other just before arriving in this northwestern shrine city on the eve of a major Christian religious festival, killing at least 77 people and injuring more than 140. (AP Photo)

Then, video graphics producer Panagiotis Mouzakis at the GraphicsBank in London had one of those brainstorm moments that remind us that journalism is about thinking –and sometimes counting, too. AP had obtained the security camera footage of the train rounding a bend, derailing and plowing into two pylons. Mouzakis realized that if he could figure out the distance between the pylons, he could calculate how fast the train was moving.

What followed was a true multiformat effort: First, Mouzakis advanced the video frame-by-frame, using the time stamp to figure out how long it took the train to travel between the first pylon and the second. Next, Madrid-based freelance cameraman Alfonso Bartolome Zafio, who was running the liveshot at the scene, estimated the distance between the two pylons. To back that up, Europe Desk editor Fisnik Abrashi used an AP photo picked up from a local newspaper to count how many cross ties there were on the tracks between the pylons, and editor Bob Barr, a railroad buff, researched the typical distance between ties on European railroads.

Each method gave AP a range of possible speeds: one was 89-119 mph; the other 96-112 mph. The speed limit was 50 mph, so AP was able to report far ahead of official announcements that the train was traveling about twice the speed limit. (Black box data released five days later showed they were exactly right: The preliminary investigation shows the driver braked from 119 mph to 95 mph in the moments before the crash.)

The resulting story was used around the world, including in The Washington Post, The Guardian, the Boston Globe and ABC News. The Daily Mail in Britain _ where news agency attribution is almost unheard of _ credited AP’s calculation, and both the BBC and Sky News cited the AP analysis in their nightly news programs.

CNN’s “Reliable Sources” highlighted the AP reporting. In his glowing account, host Frank Sesno called AP’s calculations the highlight of the week:  “In less than two hours, the AP went from video to story, providing vital information. For instincts, initiative and transparency in reporting, the AP proved itself a reliable source.”

For taking material everyone had access to, and finding a way to draw crucial conclusions from it at a speed that produced exclusive news, Mouzakis wins this week’s $500 prize.

— A memo to staff from Senior Managing Editor for U.S. News Michael Oreskes