A cardiologist who once treated former President George H.W. Bush has been fatally shot by a fellow bicyclist while riding through a Houston medical complex, and police are trying to determine if the shooting was random or a targeted act.

The shooting happened around 9am Friday as Dr Mark Hausknecht was going northbound through the Texas Medical Center, according to Houston Police Executive Assistant Chief Troy Finner.

'The suspect was on a bicycle as well. Rode past the doctor, turned and fired two shots. The doctor immediately went down,' Finner said.

Hausknecht, 65, was hit at least once and taken to a nearby hospital, where he died.

Dr Mark Hausknecht (left) was killed Friday while riding his bike in Texas. He has treated former president George H W Bush in 2000

The shooting happened around 9 am Friday as Dr Hausknecht was going through the Texas Medical Center, where he worked

Hausknecht was going in to work at Houston Methodist Hospital when he was shot, the hospital's CEO said in an email to staff.

The area where the shooting took place is part of a 1,345-acre complex of hospitals and medical institutions, including the University of Texas MD Anderson Cancer Center, and is busy with traffic and pedestrians during the day.

He had been in medical practice for almost 40 years and specialized in cardiovascular disease

Authorities late Friday continued searching for the suspect, who was wearing a gray warmup jacket, khaki shorts and a tan baseball cap.

Hausknecht treated Bush in February 2000 for an irregular heartbeat after the ex-president complained about lightheadedness while visiting Florida.

The cardiologist appeared with Bush at a news conference after his treatment.

Bush on Friday offered his condolences to Hausknecht's family.

'Mark was a fantastic cardiologist and a good man,' Bush said in a statement. 'I will always be grateful for his exceptional, compassionate care.'

Hausknecht had been in medical practice for almost 40 years and specialized in cardiovascular disease, said Marc Boom, president and CEO of Houston Methodist Hospital.

Hausknecht was part of the hospital's medical staff as well as its DeBakey Heart and Vascular Center.

'His patients appreciated his kind bedside manner and the extra time he took to answer their questions and fully explain their condition and treatment,' Boom said in an email to employees on Friday. 'Our employees who worked with him said patients were so proud to call him their doctor.'

Police are still looking for the suspect who rode past the doctor and fired two shots