Keith Emerson, one of the founding members of progressive rock group Emerson, Lake and Palmer, may have killed himself, police have said.

Sergeant Erika Aklufi of Santa Monica police said the 71-year-old keyboardist, who died at his home in California, had an apparent self-inflicted gunshot wound to the head.

"It's being investigated as a suicide," she said.

Emerson was found dead by his long-time girlfriend Mari Kawaguchi at their Santa Monica condominium on Friday morning.

Bandmate Carl Palmer said he was "deeply saddened" and paid tribute to his "brother-in-music".

"Keith was a gentle soul whose love for music and passion for his performance as a keyboard player will remain unmatched for many years to come," he said in a statement online.

"He was a pioneer and an innovator whose musical genius touched all of us in the worlds of rock, classical and jazz.

"I will always remember his warm smile, good sense of humor, compelling showmanship, and dedication to his musical craft.

"I am very lucky to have known him and to have made the music we did together.

"Rest in peace Keith."

Ms Kawaguchi said Emerson was a natural composer.

"He was just natural. The music was always in his head, always," she said. "Even when he was sleeping, you know, I could tell he was always thinking about music.

"Sometimes he would wake up and compose music. And it was all so, so beautiful."

A statement on the official Emerson, Lake and Palmer Facebook page said: "We regret to announce that Keith Emerson died last night at his home in Santa Monica, Los Angeles, aged 71.

"We ask that the family's privacy and grief be respected."

The group, keyboardist Emerson, producer Greg Lake, and drummer Palmer, formed in London in the 1970s and released seven albums together.

They parted ways in 1979 before reforming in 1991 and releasing two more albums.