SEATTLE, Wash. -

Authorities say a Metro Transit bus driver had a fatal heart attack while driving in Seattle Thursday night.

King County Department of Transportation officials say 63-year-old Sam Williams was driving on Seattle's Alaskan Way Viaduct just south of downtown when said he was having a heart attack.

Officials say he became semiconscious and that passengers came to his aid as they noticed the bus swerving at slow speeds.

Officials say one passenger helped bring the bus to a stop about six inches from the viaduct's guardrail while others helped remove Williams from his seat and began performing CPR.

Officials say a retired police officer driving behind the bus also stopped to help.

Williams was pronounced dead at a Seattle hospital.

Williams started as a part-time driver in 2010 and became full-time in 2014.