Juan Gabriel performs on stage during Volver 2015 Tour at Viejas Arena on Feb. 6, 2015 in San Diego, Calif.

Mexican superstar Juan Gabriel, the bigger than life performer who at 66 years old revamped his career in spectacular fashion, has died unexpectedly.

Juan Gabriel was in the midst of the U.S. leg of his MeXXico Es Todos tour, a 22-city arena trek that had him playing on 360 stages to accommodate sold out crowds.

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On Sunday morning (Aug. 28), sources close to him said he was in Los Angeles, where had played the Forum Friday night. As he readied to go to the airport to fly to El Paso, the next stop on his tour, he suffered a massive heart attack. An official statement from the family is expected to come.

At the time of his death, Juan Gabriel, already one of Latin music’s biggest superstars, was arguably at the apex of his career, despite his age.

He ranked at No. 18 on Billboard’s 2015 Money Makers list (just below Ed Sheeran and just above Florida Georgia Line), largely thanks to $11.6 million in touring revenue in 2015. He scored the year’s highest-grossing Latin tour and had the top-selling Latin album, Los Duo, which has moved 131,000 units, according to Nielsen Music.

The week before his death, his latest album, Vestido de Etiqueta: Por Eduardo Magallanes, debuted at No. 1 on Billboard’s Top Latin Albums chart. In the fall, Telemundo will begin airing a TV series based on his life.

While Juan Gabriel was a prodigal performer who reveled in singing three, four even five-hour concerts, his biggest claim to fame were his songs -- romantic, colloquial, emotional compositions that sometimes rambled but managed to strike a universal chord with lyrics that could apply to many people and many situations.

He was versatile and traversed pop and mariachi with ease, his songs perfectly suited for both formats. Up to the day of his death, Juan Gabriel would tour with both his pop band and his longtime mariachi.

All told, Juan Gabriel charted 31 songs on the Hot Latin Songs chart since its inception in 1986, and actually had the first No. 1 ever on the inaugural chart (dated Oct. 4, 1986) with “Yo no se que me paso.”

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Beyond his own interpretations, Juan Gabriel was covered by a slew of artists, from Rocío Dúrcal to Gloria Trevi to La India to Marc Anthony, who credits Juan Gabriel’s “Hasta que te conocí” as the inspiration behind his Latin music recording career.

“When I heard this song, I swear I saw lights, I saw clarity,” Anthony told Billboard. “I knew the song was going to change my life. That was it. I never recorded freestyle again” (Anthony’s version of Juan Gabriel’s hit reached No. 13 on the Hot Latin Songs chart, igniting sales of Anthony’s 1993 debut salsa album, Otra Nota.)

“As long as there is someone out there who sings my songs,” Juan Gabriel once said, “Juan Gabriel will live.”

Born Alberto Aguilera Valadez, the son of a farmer in Parácuaro, Mexico, about 165 miles west of Mexico City, Juan Gabriel was an unlikely star.

Producer Eduardo Magallanes, who as a record executive with RCA Mexico signed Juan Gabriel to his first recording contract in 1971, initially auditioned him a year prior and wasn’t impressed.

“He was very thin, very quiet, very observant, and his eyes shone with the desire to be heard,” Magallanes told Billboard. But he was self-taught and had a lisp. “More desire than size,” says Magallanes.

But a year later, Juan Gabriel returned, armed with an arsenal of songs that Magallanes knew could be hits. Success was immediate.

“[In the beginning] I could never have imagined what he would become with the years,” says Magallanes. “But if there were a list of the 50 best singers in the world of the past 50 years, he would be in it.”

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Although Juan Gabriel seemed capable of delivering hit after hit, his professional life was tumultuous.

Business savvy and well aware of his worth as an artist and a songwriter, despite his humble background, in the mid 1980s he embarked on a protracted legal battle with his label, BMG. It led to him refusing to record for years, until Jesus Lopez, today the chairman of Universal Music Latin America & Iberian Peninsula, took over as managing director for BMG Mexico in 1989 and reached an agreement.

Juan Gabriel didn’t forget.

When López left BMG in 1996, Juan Gabriel flew his mariachi to Miami to serenade the executive for his birthday for over 90 minutes.

“It was such a gesture,” says López. “I’ve never had a better birthday gift. We kept in touch, and in 2004, when his contract was up, I signed him to Universal.”

Although Juan Gabriel remained a prolific touring artist, his record sales dwindled in the last decade as urban music became Latin’s new pop.

However, in early 2015, he released Los Duo (The Duets), a collection of his greatest hits reimagined as duets with a broad variety of acts representing all genres of Latin music, from rock/pop star Juanes to Fifth Harmony.

The album was an instant success and dominated album sales in the U.S. and Mexico, eventually becoming the top-selling album of 2015.

In December 2015, Juan Gabriel followed up with Los Duo, Vol. 2, which was just as adventurous and also soared to the top of the charts.

At the same time, Juan Gabriel notched the highest-grossing Latin tour of 2015, raking in $31.8 million for 32 shows in the U.S. and Puerto Rico alone.

Ironically, Juan Gabriel was initially slated to go on tour in 2014, but postponed due to health reasons.

His son Ivan Aguilera, who was also his manager the last few years, told Billboard his father, who didn’t travel with a physician, stuck to a healthy eating regime, rose early and took naps to keep his energy up.

But Juan Gabriel’s landmark 2015 tour quelled any speculation about his longterm health.

“The man is a monster,” said promoter Henry Cardenas, whose Cardenas Marketing Network produced Juan Gabriel’s tours.

“Acts like Juan Gabriel are comparable to Madonna or U2 in gross, except you’re talking about a far smaller demographic. But it’s a demographic that’s willing to pay as much or more for their artist.”

Juan Gabriel leaves behind a desolate legion of fans spread around the world, and Sunday afternoon he was the top-trending Twitter topic worldwide, outpacing even the VMAs. The thousands of Twitter messages from fans illustrated his pan-generational appeal.

“My grandma would listen to Juan Gabriel while cleaning/cooking,” wrote one. “I'm glad I got to see him live once with her.”

Juan Gabriel is survived by his four children.