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Houston Police Chief Art Acevedo is asking for the public's help to locate the killer of cardiologist Mark Hausknecht.

They began searching for him after receiving an anonymous tip, police said, without providing further details.

JUST IN: Police: Arrest warrant issued for Joseph James Pappas in bicycle stalking murder of Houston cardiologist Dr. Mark Hausknecht; possible motive involves death of suspect's mother while in surgery that was performed by the doctor 20+ years ago.

This man is risky, this man is capable, this man has some skills.

Houston Mayor Sylvester Turner praised the police department for their "top notch investigators working with information provided by the public, from prominent cases to those that escape widespread attention".

Authorities on Wednesday, Aug. 1, 2018, identified a man who they believe gunned down one of former President George H.W. Bush's doctors last month as Pappas, the son of a woman who died while the doctor was operating on her more than 20 years ago.

The friend called police out of concern and when officers arrived at Pappas' home to conduct a welfare check, he was nowhere to be found.

Acevedo told reporters that evidence indicated Pappas put a lot of planning into the alleged crime.

Local officials say they do not know what may have prompted the killing so many years after Mr Pappas' mother's death.

"So it appears that this may be a 20 year-old grudge that this man held".

Both men were on their bikes on July 20 when Pappas rode past Hausknecht before turning around and fatally shooting him, police said. He said the last anyone had heard from Pappas was in a Tuesday morning text message in which Pappas wrote that he was going to kill himself.

Pappas had been in law enforcement from 1983 until he ended his service as a reserve officer in 2013, according to NBC News.

News of his death made national headlines and prompted heartfelt responses from the medical community, other local hospitals and former U.S. President George H.W. Bush, the doctor's former patient.

A suspect sketch that police later released depicted a man wearing glasses and a cap.

Cops had previously released surveillance images of the shooting, which occurred as Hausknecht was riding a bicycle to work, and shared eyewitness testimony of the shooting.

The suspect is considered armed and unsafe, and reportedly made phone calls in which he discussed committing suicide. "I will always be grateful for his exceptional, compassionate care", the statement read.