In early May 2014, as Ukraine's conflict with Russia-backed separatists in the east was exploding into all-out war, the Spanish-language channel of Russia's state-funded RT network brought on a guest to discuss the crisis.

Seemingly in his 30s or 40s, he sported a blue-and-white-striped Puma polo shirt, a five-o'clock shadow, and short brown hair slightly tousled on his forehead. RT Spanish identified the man as "Carlos," a Spaniard the network described as an air-traffic controller who "had been working in Ukraine for five years." He had agreed to speak on the condition that his name not be used, RT Spanish said. His face was blurred to guard his anonymity.

Carlos claimed to have fled Kyiv after his social-media posts prompted death threats against him from supporters of the Euromaidan protests that swept Ukraine’s Moscow-friendly president, Viktor Yanukovych, from power in February of that year. The segment featured posts from the Twitter account attributed to the guest: @spainbuca.

This man, however, was a fabulist. And two months later, the Twitter account shown in the interview would launch one of the most notorious and enduring hoaxes of the Ukraine conflict -- one that would later be cited by a top Russian military official and, ultimately, President Vladimir Putin himself.

On July 17, 2014, Malaysia Airlines Flight MH17 was shot down over territory held by Russia-backed separatists in eastern Ukraine, killing all 298 people on board. Immediately after the tragedy, @spainbuca purported to be watching the events unfold from an air-traffic control tower at Kyiv's Boryspil Airport. It posted a string of dramatic claims suggesting Ukraine had shot down the plane and was attempting a cover-up.

One claim in particular ricocheted across Russian state media and Twitter: that two Ukrainian fighter jets had flown close to the Boeing 777 shortly before it disappeared from the radar.

But red flags about @spainbuca's credibility emerged quickly as well. Why was a Spanish air-traffic controller working in Kyiv? Where were the reports corroborating his claims? Why was he back in Ukraine after telling RT Spanish that he'd fled the country?

No evidence ever emerged that such a Spanish air-traffic controller actually worked at Boryspil, and @spainbuca was suspended by Twitter shortly after its MH17 tweets. Journalists and social-media users widely dismissed "Carlos" as a ruse. Still, some conspiracy theorists insisted his story was real and that he had either been targeted by Western intelligence services or gone into hiding. Others claimed he was part of the notorious Russian troll farm eventually indicted in the United States for alleged meddling in U.S. elections. One Ukrainian official has suggested the Carlos persona was a Russian intelligence operation.

The claims by Carlos about MH17 and his professional background were indeed a hoax.

But a niggling question remained: If Carlos was a fake, who was the man in the RT Spanish interview?

Today, RFE/RL and the RISE Project, a Romanian investigative-journalism group based in Bucharest, can reveal that he is a Spanish ex-convict detained by Romanian police for alleged fraud in August 2013 -- nine months before his appearance on RT Spanish.

We also found substantial evidence linking this man to the @spainbuca account that posted the bogus MH17 tweets.

Our investigation tracked Carlos through the Spanish courts and the Romanian criminal-justice system, to the posh Bucharest neighborhood where he lived, and even to a Romanian cooking show he appeared on in 2015 -- when some people were still positing that he was in hiding for his alleged MH17 whistle-blowing.

Ultimately, we made contact with someone credibly claiming to be the very same Carlos in the RT Spanish interview -- and who spun fresh provocative claims about the bizarre role he played in the Ukraine conflict.