(CNN) A cardiologist who treated former President George H.W. Bush was killed Friday morning in a bicycle-to-bicycle drive-by shooting near Texas Medical Center in Houston, authorities said.

Dr. Mark Hausknecht and the shooter were both riding bikes on South Main Street, near Texas Children's Pavilion for Women, shortly before 9 a.m. local time, Executive Assistant Police Chief Troy Finner said at a news conference.

Hausknecht, 65, was biking north when he passed the shooter going in the other direction, Finner said. The shooter turned, fired two shots at Hausknecht and rode away on his bike, Finner said.

Hausknecht was taken to a hospital, where he was pronounced dead, Finner said.

Finner said investigators didn't know if the shooting was targeted, random or caused by road rage. He said a few people witnessed the shooting and video may become available.

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