John Entwistle, bassist for the legendary rock band the Who, died in his sleep Thursday morning (June 27) in his room at the Las Vegas Hard Rock Hotel and Casino. He was 57.

Entwistle's body was discovered by his road assistant at around noon local time, said his manager Steve Luongo. The cause of death is still under investigation, but Luongo said it is believed that the bassist died of a heart attack.

While Entwistle had a heart condition, he seemed to be in good health and his death came as a complete surprise, Luongo said.

The Who had rehearsed together Wednesday night in Las Vegas and were due to launch a tour there Friday night. The bassist was also scheduled to attend a show of his artwork in Grammy's Art of Music Gallery at the Aladdin Desert Passage Shops the afternoon he died.

The band's North American outing would have been the continuation of a triumphant reunion that began in 2000 with a highly successful world tour. The renewed buzz about the band reached a peak last year when the Who became the hands-down highlight at the Concert for New York City (see [article id="1450204"]"McCartney, Jagger, Bowie, The Who Come To NY's Aid"[/article]). All the tension and energy of the moment was galvanized in the band's explosive set, which included hits like "Won't Get Fooled Again" and "Who Are You."

Encouraged by their recent success, the band were planning to return to the studio in October to work on their first new material since 1982's It's Hard (see [article id="1449484"]"Who Hope Onstage Magic Translates To Studio"[/article]). In an interview with MTV News late last year, Entwistle said, "We're working so well as a five piece. We want to try to carry that on the album somehow. We're eager to see what happens."

The Who's last shows with Entwistle were on February 7 and 8 at London's Royal Albert Hall.

Entwistle, considered to be one of the greatest rock bassists of all time, was familiar to fans for his sturdy basslines and deadpan performance style. While vocalist Roger Daltrey bounded around the stage and swung his microphone like a lasso and guitarist Pete Townshend windmilled, Entwistle would stand stock-still and expressionless. His passages were especially important to the band's music because he held down the rhythms with flair while drummer Keith Moon flailed away.

The bassist also sang and wrote several of the band's deep tracks and B-sides, including "Silas Stingy," "Whiskey Man," "My Wife," "Trick of the Light," "The Quiet One," "Heaven and Hell" and the quirky "Boris the Spider," perhaps his best-known Who song.

"We lost the Jimi Hendrix of bass guitar," said manager Luongo.

In addition to his work with the Who, Entwistle recorded seven studio albums, beginning with 1971's Smash Your Head Against the Wall and ending with 2000's Music From Van-Pires. He was the first member of the band to release a solo LP.

With his solo material, Entwistle more deeply expressed his morbid side and his dark sense of humor, which was keenly depicted in the graveyard imagery on the cover of his third solo album, Rigor Mortis Sets In. He sometimes wore a skeleton jumpsuit onstage and was never seen without his trademark spider pendant.

Born in Chiswick, London, Entwistle was accomplished in both the piano and French horn. At age 15, he became bassist for the Confederates, a grammar-school skiffle group that also included guitarist Townshend. Impressed with his deft musicianship, Daltrey plucked Entwistle for his group, the Detours, with Townshend following his lead soon after. Drummer Keith Moon eventually rounded out the lineup, and the group renamed themselves the High Numbers, and finally the Who. The band, a frontrunner of the second wave of British Invasion groups, released their debut album, The Who Sings My Generation, in 1965.

After three albums of solid, R&B-based rock and a 1968 compilation, Magic Bus,, the Who shattered convention with the ambitious rock opera Tommy in 1969. Other landmark releases followed. 1970's Live at Leeds is widely considered to be one of the best live albums ever recorded. It was followed the next year by Who's Next, which included the hits "Behind Blue Eyes" and "Baba O'Riley." The band returned to an epic songwriting style in 1973 with Quadrophenia, a concept album about feuding sects of mods and rockers.

Tragedy struck the band in 1978 when Moon died of an accidental drug overdose just weeks after the release of the album Who Are You. He was replaced by former Small Faces drummer Kenney Jones. Two more studio albums followed. Then in 1982 the band embarked on what they billed their farewell tour, which was documented on the live album Who's Last in 1984.

The band reunited a year later for Live Aid, and in 1989 they staged a 25th anniversary reunion tour. A period of dormancy followed until 1994 when the band again reformed, this time for two concerts to celebrate Daltrey's 50th birthday. The band performed a number of one-off shows of Tommy and Quadrophenia in the years that followed before staging a full-scale comeback tour in 2000.

[This story was updated on 06.27.02 at 9:15 p.m. ET.]