Keith Emerson in 2015. (Paul A. Hebert/Invision/AP)

Keith Emerson, founder and keyboardist of the progressive-rock band Emerson, Lake and Palmer, was found dead March 11 at his condominium in Santa Monica, Calif. He was 71.

His partner, Mari Kawaguchi, confirmed the death but declined to disclose further details. The music trade publication Billboard, citing police confirmation, said that the cause was a gunshot wound to the head and that the death is being investigated as a possible suicide.

Mr. Emerson, drummer Carl Palmer and vocalist-guitarist Greg Lake were giants of progressive rock in the 1970s, recording six platinum-selling albums.

They and other hit groups such as Pink Floyd, the Moody Blues, Yes and Genesis stepped away from rock’s emphasis on short songs with dance beats, instead creating albums with ornate pieces full of complicated rhythms, intricate chords and time-signature changes. The orchestrations drew on classical and jazz styles and sometimes wedded traditional rock instruments with full orchestras.

Emerson, Lake and Palmer’s 1973 album “Brain Salad Surgery” included a nearly 30-minute composition called “Karn Evil 9” that featured a Moog synthesizer and the eerie, carnival-like lyric, “Welcome back my friends, to the show that never ends.”

Although it filled stadiums, ELP also was ridiculed as the embodiment of the pomposity and self-indulgence that rock supposedly stood against. When the punk movement took off in the mid-1970s, the band was a special target, openly loathed by the Sex Pistols’ Johnny Rotten, among others.

Years later, Rotten, whose real name is John Lydon, and Mr. Emerson became friends.

“He’s a great bloke,” Lydon said in 2007. “I’ve told Keith in no uncertain terms that what put me off his band were those 20-
minute organ solos and that film of their convoy of trucks crossing America.”

Keith Noel Emerson was born in Todmorden, Britain, on Nov. 2, 1944, and grew up in Worthing. He showed gifts on the piano as a child and would later cite jazz greats such as Fats Waller, Oscar Peterson and Dave Brubeck as early influences.

By his late teens, Mr. Emerson was playing in blues and jazz clubs in London. He helped form one of the first progressive-rock groups, the Nice, before hooking up with Lake and Palmer in 1970 and debuting with them at the Isle of Wight Festival, shows that also featured Jimi Hendrix and the Who.

ELP broke up in 1979, reunited in 1991, later disbanded again and reunited one last time for a 2010 tour.

Mr. Emerson wrote a memoir, “Pictures of an Exhibitionist,” that explored some of the hedonistic aspects of his life.

A complete list of survivors could not immediately be confirmed.

Throughout his life, he continued to compose and perform, sometimes solo and at other times with various musicians, including Lake.

Mr. Emerson had been composing and working with internationally known symphony orchestras, including in Germany and Japan, and was about to embark on a short tour in Japan starting April 14 with his band, Kawaguchi said. His work included a classical piano concerto.

“All these people from the classical world were playing his music,” she said. “When he was young, he was using classical music for rock, and now the wheel has turned and now the classical world is using his compositions.”

Despite his influence, Mr. Emerson never considered himself a rock or pop titan, and his true musical devotion lay elsewhere.

“At home he . . . listened to either classical or jazz,” Kawaguchi said. “We never listened to rock.”