Bobby Hatfield, one half of the Rock and Roll Hall of Fame duo the Righteous Brothers, died of undetermined causes in a Michigan hotel room Wednesday night at age 63.

Hatfield was found dead in his hotel room just a half-hour before he and singing partner Bill Medley were slated to perform at Miller Auditorium on the campus of Western Michigan University in Kalamazoo, Michigan.

As police officers did not see any evidence of unusual circumstances or foul play, it appears that Hatfield died of natural causes, according to a spokesperson for the Kalamazoo Public Safety office. He was pronounced dead at 7:02 p.m. and an autopsy is scheduled for Thursday (November 6).

The Righteous Brothers, inducted into the Hall of Fame this year by Billy Joel, were known for such huge "blue-eyed soul" hits as "You’ve Lost That Lovin’ Feelin’ " and the haunting "Unchained Melody."

According to reports, Hatfield had been asleep in his hotel room for much of Wednesday and did not answer a 6 p.m. wake-up call. Shortly after, hotel staff and authorities entered his room and found his body.

Robert Lee Hatfield was born on August 10, 1940, in Beaver Dam, Wisconsin. His family moved to Anaheim, California, when he was 4 and Hatfield later became active in singing groups in high school and in college at Long Beach State University.

Both Hatfield and Medley were journeyman soul singers in California before they hooked up as part of the group the Paramours in 1962. They formed the Righteous Brothers a year later. The match was perfect, with Medley singing the baritone parts and Hatfield using his crystal-clear high-tenor voice to sing falsetto notes. They had early success with the song "Little Latin Lupe Lu," but it wasn’t until they paired with "Wall of Sound" producer Phil Spector for their first album on his Philles label that they became internationally known.

The sweeping 1965 ballad "You’ve Lost That Lovin’ Feelin’ " was unusual for the times, clocking in at almost four minutes when the popular British Invasion bands of the day opted for shorter, high-energy tunes. But the power of the soaring, orchestral track and the combination of Medley and Hatfield’s voices pushed it to #1. The song, which was famously crooned by Tom Cruise and Anthony Edwards in the film "Top Gun," went on to become one of the most frequently played songs in the history of American radio with more than 8 million spins, according the group’s official Web site.

Their association with Spector continued to spawn hits, including 1965’s "Just Once in My Life," "Ebb Tide" and the classic "Unchained Melody." The pair split with Spector that year and had another hit, "(You’re My) Soul and Inspiration," on their new label, Verve, but the hits would soon dry up. By 1968, Medley had left for a solo career and Hatfield kept the Righteous Brothers going with the addition of singer Jimmy Walker.

The pair reunited in 1974 and had a hit with their tribute to fallen rockers, "Rock & Roll Heaven." Medley retired from performing for the rest of the '70s, but the duo began a lucrative career performing as part of oldies shows for much of the '80s and '90s.

After a two-week run of dates in Las Vegas in October, the Kalamazoo gig was to be the first of a string of five Midwestern shows the pair were scheduled to perform before making their way back to Vegas for another two weeks of dates at the Orleans Hotel and Casino.