Sir George Martin, the Beatles’ longtime producer and often referred to as the “fifth Beatle,” has died at age 90. The cause of death is still unknown.

Ringo Starr broke the news on Twitter. “God bless George Martin,” he wrote, “peace and love to his family.” He later added, “Thank you for all your love and kindness.”

Paul McCartney wrote a moving tribute to the late producer on his website, calling Martin a “true gentleman” and “like a second father”:

I’m so sad to hear the news of the passing of dear George Martin. I have so many wonderful memories of this great man that will be with me forever. He was a true gentleman and like a second father to me. He guided the career of The Beatles with such skill and good humour that he became a true friend to me and my family. If anyone earned the title of the fifth Beatle it was George. From the day that he gave The Beatles our first recording contract, to the last time I saw him, he was the most generous, intelligent and musical person I’ve ever had the pleasure to know.

Martin was born in London in 1926. After serving in the Navy during World War II, he landed a job with BBC’s Classical Music Department before transitioning to a position with EMI’s imprint Parlophone. There, he produced a number of a comedy records.

Martin met the Beatles in 1962. After listening to a demo tape, the producer offered the band their first record contract. He went on to produce much of their catalog, including their 1963 debut Please Please Me all the way through 1969’s Abbey Road. (He notably did not helm the Phil Spector-produced Let It Be.) Martin significantly contributed to the Beatles’ songs, adding strings to “Yesterday” and “Eleanor Rigby,” piano to “In My Life” and inspiring the tape loops heard on “Tomorrow Never Knows.”

Martin went on to work with McCartney and Starr in their solo efforts. He’s also produced albums for Cheap Trick, Jeff Beck, Kate Bush, Elton John and many more. He’s won six Grammys and an Oscar for his contributions to A Hard Day’s Night.

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