By Eric Levenson, CNN

(CNN) -- Houston police released an artist's sketch and surveillance images of a cyclist who they say shot and killed Mark Hausknecht, an acclaimed cardiologist who treated former President George H.W. Bush.

Hausknecht, 65, and the suspect were both seen riding bicycles on South Main Street on Friday in Houston, police said. Hausknecht was biking north when he passed the shooter going in the other direction, Executive Assistant Police Chief Troy Finner said. Then the shooter turned, fired two shots at Hausknecht and rode away on his bike, Finner said.

Finner said investigators did not know if the shooting was targeted, random or caused by road rage.

In surveillance images released by Houston police on Twitter, Hausknecht is circled in green and the suspect is circled in red. The shooting took place moments after and one block away from when the images were taken, police said.

Police also released a composite sketch of the suspect in the killing and asked for tips on his identity.

The suspect was described by witnesses as a white or Hispanic man, about 30 years old and about 5-foot-10 with a slender build. He had a tan baseball cap, sunglasses and was clean shaven, and was wearing a gray warmup jacket and khaki shorts, police said.

Hausknecht was a cardiologist at Houston Methodist Hospital and had treated former President Bush, 94.

Jim McGrath, a spokesman for Bush, offered the 41st president's condolences on the killing.

"President George HW Bush was deeply saddened by the tragic circumstances surrounding the untimely passing of Dr. Mark Hausknecht, and 41 sends his most sincere condolences to the Hausknecht family, his colleagues at Houston Methodist, and his friends," McGrath said.

"Mark was a fantastic cardiologist and a good man," Bush said in the statement. "I will always be grateful for his exceptional, compassionate care. His family is in our prayers."

Houston Methodist Hospital said Hausknecht has been in practice for almost four decades and was an important member of the hospital staff. He was a leader in the Houston Cardiovascular Associates and specialized in cardiovascular disease. He received his medical degree from Baylor College of Medicine, the hospital said.