A doctor who treated former US president George H W Bush may have been shot dead out of revenge, police have said.
Police are searching for Joseph James Pappas in Houston and have warned he should be considered armed, dangerous and possibly suicidal.
Pappas' mother died while being treated by cardiologist Mark Hausknecht more than 20 years ago.
Dr Hausknecht was shot three times on 20 July while riding his bicycle to work at the Texas Medical Center.
He and Pappas were both seen riding their bicycles in the area shortly before the shooting.
The gunman rode past the doctor before turning around to kill him, police say.
Houston police chief Art Acevedo said: "There was a lot of planning that went into this.
"There was a lot of planning and, sadly, some skill."
Police received a tip on Tuesday after CCTV was released of the suspect.
An arrest warrant was issued for Pappas who police say may still have his 10-speed bicycle.
Pappas was not home when officers went there on Wednesday but Mr Acevedo said they found evidence linking Pappas to the killing.
He gave no further details.
Pappas has not been heard from since Tuesday morning, when he texted someone to say he was going to kill himself.
Dr Hausknecht, whose body was found near a construction site, treated George H W Bush for an irregular heartbeat in 2000.
The ex-president had complained about light-headedness in 2000.
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Friends and family have described Dr Hausknecht as a humble and generous man who was adored by his patients, volunteered in his community and cared about the environment.
He was also an avid cyclist and rode his bike to work each morning.
A doctor who treated former US president George H W Bush may have been shot dead out of revenge, police have said.
Police are searching for Joseph James Pappas in Houston and have warned he should be considered armed, dangerous and possibly suicidal.
Pappas' mother died while being treated by cardiologist Mark Hausknecht more than 20 years ago.
Dr Hausknecht was shot three times on 20 July while riding his bicycle to work at the Texas Medical Center.
He and Pappas were both seen riding their bicycles in the area shortly before the shooting.
The gunman rode past the doctor before turning around to kill him, police say.
Houston police chief Art Acevedo said: "There was a lot of planning that went into this.
"There was a lot of planning and, sadly, some skill."
Police received a tip on Tuesday after CCTV was released of the suspect.
An arrest warrant was issued for Pappas who police say may still have his 10-speed bicycle.
Pappas was not home when officers went there on Wednesday but Mr Acevedo said they found evidence linking Pappas to the killing.
He gave no further details.
Pappas has not been heard from since Tuesday morning, when he texted someone to say he was going to kill himself.
Dr Hausknecht, whose body was found near a construction site, treated George H W Bush for an irregular heartbeat in 2000.
The ex-president had complained about light-headedness in 2000.
More from Texas
Friends and family have described Dr Hausknecht as a humble and generous man who was adored by his patients, volunteered in his community and cared about the environment.
He was also an avid cyclist and rode his bike to work each morning.
