Eyewitness News has found a number of ads placed in gun forums online linked to Pappas.
Pappas, 62, was charged Wednesday for the July 20 murder of Dr. Hausknecht as he rode his bicycle to work in the Texas Medical Center.
Houston police say tips came in on Tuesday afternoon led them to Pappas. Houston Police Chief Art Acevedo says Pappas' mother died during surgery more than 20 years ago, and they believe that is the motive in the murder of Dr. Hausknecht.
At 4 a.m. Wednesday, police performed a search warrant for Pappas. He was not found at home when police showed up and has since not been located.
WATCH: HPD announces charges against Joseph Pappas
Suspect announced in doctor's murder
Anyone with information on Pappas' whereabouts is urged to contact the Houston Police Department Homicide Unit at 713-308-3600 or Crime Stoppers at 713-222-TIPS.
He is considered armed and dangerous, and police believe he may be suicidal.
On Monday, Houston police tweeted out video of the suspect in a nearby neighborhood immediately following the doctor's shooting.
Houston police release new surveillance video of suspect in doctor's murder
SEE ALSO: Mark Hausknecht seen in new surveillance photos with suspect who shot and killed him
New surveillance video shows moments before Houston doctor was killed
New video shows suspect in doctor's murder riding through neighborhood after shooting
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