DALLAS — When Thomas Johnson enters a Dallas County courtroom on Monday, whether or not he committed murder will take a backseat to his state of mind at the time.

His defense attorneys have entered a “not guilty by reason of insanity” plea as testimony gets underway this week.

The 25-year old’s murder trial starts three-and-a-half years after Dallas Police say Johnson borrowed a stranger’s cell phone and confessed to 911 operators that he killed a man.

Dave Stevens was a regular jogger along White Rock Trail.

It was there, detectives say, Johnson, seemingly at random, attacked the 53-year-old mechanical engineer repeatedly with a machete.

Stevens could only be identified by fingerprints.

George Milner III is a Dallas based criminal defense attorney that is not associated with this case.

He said an insanity defense gets a lot of attention but is not commonly attempted.

"It's extremely rare," Milner said. "It sounds like in this case – everybody involved believes the man really is insane.”

Johnson was found incompetent to stand trial in April 2016 and was sent to the North Texas State Hospital in Vernon for six months for treatment.

He was found competent to stand trial in June 2018.

"The jury is going to be moved by the horrific nature of the crime," Milner said.

To present an insanity defense in Texas, a defendant must have had a mental disease or defect that caused the defendant not to know that his or her conduct was wrong at the time of the offense.

Family said Johnson was diagnosed with schizophrenia in 2014, two years after the Skyline High graduate and Texas A&M wide receiver went missing from College Station during his freshman season.

Johnson was located wandering a street in Dallas a few days later, rambling.

A family member would accuse him of stealing a few years later – and then the horrific attack in October 2015 happened. The attack shocked the Dallas running community. 

Patti Stevens, grief stricken over the loss of her husband of more than 25 years, died by suicide two weeks after his death.

Now a jury will have to decide whether to send Johnson to prison – or likely spend the rest of his life in a secure mental health facility.