(CNN) A 71-year-old man working for the rideshare service Lyft was fatally shot, police say, while driving in Phoenix on Sunday -- his 52nd wedding anniversary.

Investigators believe that the gunshot came from outside Harold Treadwell's car at 12:32 a.m., Phoenix Police spokeswoman Sgt. Margaret Cox said.

There were no passengers in the car at the time, according to Lyft and Phoenix Police.

Cox said that police do not have any information about a suspect and that the investigation is ongoing.

Treadwell had just spoken to his wife, Frances, before the shooting, according to a GoFundMe site for his funeral expenses.

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