Houston(CNN) There is a "high probability" a cardiologist who treated former President George H.W. Bush was targeted when he was fatally shot last week riding his bicycle to work, Houston's police chief said Wednesday.
But Chief Art Acevedo emphasized that investigators are still not certain of the motive.
"There is a high probability the doctor was targeted," the chief said in a tweet from Houston police.
The suspected gunman remains at large. In a brief interview, Acevedo said he is confident police will be able find the suspect and make an arrest in the death of Dr. Mark Hausknecht because there were several witnesses in the area at the time of Friday's shooting.
Acevedo declined to say why he believed the cardiologist was likely targeted. The FBI's Houston bureau is helping city police with the investigation.
Hausknecht, who worked at Houston Methodist Hospital, and the suspect were seen shortly before the shooting riding bicycles on South Main Street, police said. Surveillance footage shows the suspect following the physician as both pedaled north, police said.
Georgia Hsieh, the wife of the renowned cardiologist, said police told her that her husband was riding through a construction area when the gunman rode up in front of him and fired three shots while facing him.
A close-up surveillance image shows the suspect wearing a blue, short-sleeve polo shirt, a khaki or white cap, khaki shorts and an olive-green backpack, police say.
Other surveillance images show the suspect just behind Hausknecht as both head north on Main across West Holcombe Boulevard, police say. In the images posted on Twitter, police highlighted Hausknecht with a green circle and the suspect with a red circle.
The shooting happened moments after that crossing, one block away, police say.