An attorney plans to seek a competency hearing for a 19-year-old Oklahoma murder suspect whose brother told investigators he said he killed their parents because he thought they were Satan worshippers communicating with him telepathically.

Michael Elijah "Eli" Walker was arraigned Wednesday on first-degree murder charges in the March 4 shooting deaths of Michael Logan Walker, 50, and Rachel Walker, 44, at their home in the Oklahoma City suburb of Edmond.

Walker's attorney, Derek Chance, who entered a not guilty plea on his client's behalf, said he would expect to seek a not guilty by reason of insanity defense if he isn't found incompetent for trial. Chance acknowledged such defenses have rarely succeeded in Oklahoma murder trials.

"He's very mentally ill, and even at this point it's difficult even for us to get through to him in any of our meetings," Chance said.

Michael Logan Walker's sister, Maya Walker, made a brief statement to reporters Tuesday following funeral services for the slain couple, alluding to her nephew's mental difficulties.

"Please see it soon enough before it happens to your family because all of us now in hindsight are realizing the pieces we're putting together now, we didn't recognize," she said.

"We hope and pray for the best possible outcome (and) for Eli, my nephew, to get the best care possible," Walker said.

It has not been revealed whether Walker had been diagnosed with, or was undergoing treatment for, a mental illness.

"We have not confirmed that yet as an official medical diagnosis, not saying it isn't true," police spokeswoman Jenny Wagnon said. "That's something we will likely be asked to do by the DA's office."

Wagnon said two pistols and an AR-15 rifle found in the home shortly after the slayings were purchased by one of the parents but declined to say which parent.

Easy access to guns only slightly increases the risk by the mentally ill of gun violence compared to the overall population, said Dr. William Carpenter Jr., a psychiatric professor at the University of Maryland School of Medicine.

"Sure, violence can be a problem ... gun violence, but with the amount of gun violence in this country it just sticks out a little bit," Carpenter said of violent acts committed by the mentally ill. "Most of the increase relates to before they're diagnosed or treated, or they're not taking their medication at the time the violence occurs."

Wagnon said police were also trying to determine where materials were acquired for four small homemade bombs found in the home and later detonated by a bomb squad.

"We'll be going through phones and computers, looking for receipts to try to find out how they were purchased ... that's part of our investigation," Wagnon said.

Meredith Davis, a spokeswoman for the Bureau of Alcohol, Tobacco, Firearms and Explosives, said the agency is aware of the case and available to assist if asked.

The insanity defense was successful for Christian Costello, 30, who a judge ruled in April 2018 was not guilty by reason of insanity in the 2015 stabbing death of his father, state Labor Commissioner Mark Costello.

In that case, prosecutors agreed with defense attorneys that Christian Costello was incompetent and both said it was unlikely he would ever be released.