Walter Becker, the guitarist and co-founder of the influential band Steely Dan, has died at age 67.

Becker was lead guitarist of the sophisticated, jazz-flavored Steely Dan, which he formed with Donald Fagen, the keyboardist and lead vocalist.

His website said he died on Sunday. No cause of death was given, but Becker was believed to have been ill for several months.

In its heyday in the 1970’s, the band scored hits with Reelin’ in the Years, Do It Again, Rikki Don’t Lose That Number and Deacon Blues.

Becker’s longtime collaborator, Fagen, said in a lengthy and heartfelt statement: “Walter Becker was my friend, my writing partner and my bandmate since we met as students at Bard College in 1967.”

“He was smart as a whip, an excellent guitarist and a great songwriter. He was cynical about human nature, including his own, and hysterically funny,” Fagan said in the statement published by Entertainment Weekly.

“I intend to keep the music we created together alive as long as I can with the Steely Dan band,” he added.

After a brief period working as songwriters and backing musicians in New York, Becker and Fagan relocated to California in 1971 and formed Steely Dan, taking the name from a sex toy in the William S. Burroughs’ novel Naked Lunch.

Steely Dan’s 1972 debut album, Can’t Buy a Thrill, featured the radio hit Do It Again and the follow-up single Reelin’ in the Years, introducing the world to the band’s musically sophisticated sound.

“I’m not interested in a rock/jazz fusion,” Becker told Rolling Stone in a 1974 interview.

“That kind of marriage has so far only come up with ponderous results. We play rock ’n’ roll, but we swing when we play. We want that ongoing flow, that lightness, that forward rush of jazz.”

Becker and Fagen remained the band’s core members as they constantly rotated musicians until disbanding in 1981 before reuniting in 1993.

Steely Dan released the album Two Against Nature in 2003, winning four Grammy awards including album of the year.

The band was inducted into the Rock and Roll Hall of Fame in 2001 and released its final album, Everything Must Go in 2003.

– With agencies