A cardiologist who once treated former US president George HW Bush was shot dead while riding a bicycle in the city's medical centre, authorities say.

Mark Hausknecht, 65, was killed on Friday morning (local time) by a man thought to be in his 30s who fired at least two shots at him from a passing bicycle as the doctor rode to work at the centre, which is home to several hospitals and medical offices, police said.

The suspect then rode off, according to police, who have yet to give a motive for the daylight shooting in the busy city area.

"Mark was a fantastic cardiologist and a good man," Mr Bush, 94, said in a statement.

"I will always be grateful for his exceptional, compassionate care. His family is in our prayers."

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Dr Hausknecht had worked in Houston for about three decades and often cycled to work, local media reported.

He once treated the former president for an irregular heartbeat.

The doctor's widow, emergency physician Georgia Hsieh, called his killing senseless, according to a statement published by the Houston Chronicle.

"When students cannot go to school without fear, and teachers need to arm themselves, what has this country come to?," Dr Hsieh's statement said.

"As a trained emergency medicine physician, I am no stranger to the devastating consequences of both intentional and accidental firearm use.

"Now my family and I have unwittingly joined the ranks of thousands of other grieving American families who have lost innocent loved ones."

Houston Police said they were studying surveillance video and following up on tips.

Police also released a composite sketch of the suspected shooter.

"This is a crime that doesn't make sense," Houston Police Chief Art Acevedo said.

"We don't even have a motive yet."

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Reuters/ABC