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.

A sketch of a suspect in the death of Dr. Mark Hausknecht

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.

UPDATE #5: Chief @ArtAcevedo states "There is a high probability the doctor was targeted." No other information is available, as additional details are part of the ongoing investigation. Anyone with information on this case is asked to contact HPD at 713-308-3600 #hounews — Houston Police (@houstonpolice) July 26, 2018

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.

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