Houston (CNN) The suspect in the slaying of Texas sheriff's Deputy Darren Goforth was found mentally incompetent in 2012 to stand trial on a felony assault charge, a prosecutor said Monday.

About three years ago, Shannon J. Miles was charged with aggravated assault with a deadly weapon after fighting a man at an Austin homeless shelter over the TV remote control, said Joe Frederick, a prosecutor in Travis County, Texas.

Miles kicked and punched the victim, injuring his face, back and head, Frederick said. The deadly weapon used in the assault was his hands.

Before trial, Miles was found mentally incompetent and sent to North Texas State Hospital in Vernon for six months, Frederick said. He was found to be competent and sent back to Travis County for trial, but prosecutors could not find the assault victim and the case was dropped, Frederick said.

Miles is charged with capital murder in the killing of Goforth, who was pumping gas into his patrol car Friday when a man approached from behind and shot him 15 times, a prosecutor said Monday.

Miles, 30, arrived Monday at Harris County District Court in Houston wearing the yellow jumpsuit the county assigns to high-security inmates, his wrists and ankles shackled to his waist.

He did not speak much during the hearing, but at one point, Judge Denise Collins admonished Miles to address her as "ma'am."

One of Miles' court-appointed attorneys, Anthony Osso, said Miles "looked to me to have a blank expression, which is always a cause for concern."

Shannon J. Miles is accused of killing Deputy Darren Goforth.

Miles will undergo a psychological examination as part of his background investigation, Osso said. A defense team is being put together that will include forensic experts and psychological experts, he said.

Harris County prosecutors have already filed a request with the county psychiatric center for documents about Miles' treatment at the center.

Osso said Miles told police that he wasn't involved in the shooting and plans to plead not guilty. Miles is being held in an isolation cell, the attorney said.

President Obama made a condolence call Monday to Goforth's widow. Obama said he told Kathleen Goforth that he would continue to "highlight the uncommon bravery that police officers show in our communities every single day. They put their lives on the line for our safety.

"Targeting police officers is completely unacceptable -- an affront to civilized society."

What led police to Miles

Prosecutors revealed details about the slaying Monday.

Goforth was found in a pool of blood next to his patrol car, which he had been filling up with gas at the time of this death, District Attorney Devon Anderson said.

Surveillance video of the shooting shows Miles wearing a white T-shirt, red shorts and tennis shoes as he walks up behind Goforth, Anderson said.

"He puts a gun to the back of his head and shoots," Anderson said, describing the video. Even when Goforth hits the ground, Miles "continues to unload his gun," she said.

"The gun holds 14 in the magazine and one in the chamber," Anderson told reporters after the hearing. "He unloaded the entire pistol into Deputy Goforth."

Miles then drove away in a red pickup truck with a white cooler in the bed, Anderson said. A search for red Ford Ranger pickup trucks yielded a result in the same 77095 Houston ZIP code where the shooting occurred.

Investigators went to the home and found what they believed to be the same red truck in the driveway. Not only did it have an aftermarket trailer hitch like the suspect vehicle, but there also was evidence it had been carrying a cooler in its bed, Anderson said.

Police knocked on the door, and a man answering the door said the truck belonged to his brother, who had just left with their mother, Anderson said.

While police were searching the home to see if Miles was there, Miles and his mother returned, Anderson said. Miles told officers the truck belonged to him, and when asked if he had any firearms, he acknowledged having a .40-caliber in a blue baseball bag in the garage, she said.

Officers got a search warrant and recovered the gun, which ballistics testing indicated was linked to the shell casings found at the crime scene, she said.

Miles is cooperating with police, but Anderson declined to comment on his possible motive, specific comments Miles made or where and how he might have obtained the gun.

Targeted for being policeman?

As far as authorities can tell, the only target on Goforth's back was a law enforcement uniform.

"This rhetoric has gotten out of control," said Goforth's boss, Harris County, Texas, Sheriff Ron Hickman. "We've heard 'black lives matter,' 'All lives matter.' Well, cops' lives matter, too. So why don't we just drop the qualifier, and just say 'Lives matter,' and take that to the bank?"

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But whether the outrage over perceived police brutality against African-Americans played a role in the seemingly random killing of Goforth is up for debate.

In response to the shooting, Chuck Canterbury, the president of the Fraternal Order of Police, announced Sunday he was renewing his call to make killing a law enforcement officer a hate crime.

Hickman acknowledged that the motive for his deputy's death has not been determined.

Osso, the defense lawyer, didn't want to talk about Miles possibly having a grudge against police.

"We're going to stay away from that," he said. "We're not looking to make it a race issue. We need to focus on the facts in this case so we're going to avoid those outside forces."

Rhetorical debate

While some lauded the sheriff for saying anti-police rhetoric has gotten "out of control," others said he prematurely linked Goforth's death to the "Black Lives Matter" movement.

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"There's no evidence that there's a connection between this rhetoric, or this sort of national discourse ... and what happened," CNN political commentator Marc Lamont Hill said. "It's an awful tragedy, but I don't think it's connected."

CNN law enforcement analyst Harry Houck disagreed, saying the anti-police rhetoric is undeniable. He cited a protest in Minnesota over the weekend in which demonstrators held a banner saying "Black Lives Matter" and chanted, "Pigs in a blanket! Fry 'em like bacon!"

Texas state Rep. Garnet Coleman said it was "unfortunate" the sheriff tied Goforth's death to the "Black Lives Matter" movement -- or anything that has to do with police brutality -- so quickly.

But the sheriff had a point, he said.

"In general, I think the overall rhetoric is overheated, and it only leads to more injury and death," Coleman said Monday. "We're going to have to figure this out, whether it's training for police or having a sit-down to decrease the rhetoric -- particularly if it discusses violence at all."

Ambush killings of cops on the rise

NYPD officers carry the casket of slain officer Wenjian Liu on January 4.

National law enforcement leaders are renewing efforts to have police officers added as a protected class under the federal hate crime laws.

Twenty-three police officers have been killed by gunfire this year, with three targeted because they were officers, Canterbury said. He included Goforth in that count. In 2014, nine officers were ambushed and killed.

"They were killed because their murderers had one purpose -- to kill a cop," he said.

Outpouring of support

Mourners created a memorial at the gas station where Goforth was killed.

If there was any hostility toward Goforth because of his job, it was quickly outweighed by the support of thousands of strangers.

As of Sunday night, donors dropped off more than $75,000 for the Goforth family at the gas station where the deputy was killed, organizer Brian McCullar said. A gofundme.com page has raised nearly $100,000.

"This was a very senseless murder and no one, be it of any race, deserved to die like this," Kimberly Bourda Murphy wrote on the page. Goforth's life "did matter, just as all lives matter."