DALLAS (CBSDFW.COM) – This morning the trial began for the man accused of murdering a runner in East Dallas. Police say Thomas Johnson confessed to the 2015 crime, but it has taken this long for the case to go to trial.

It was in October of 2015 when police say Johnson, who was 21-years-old at the time, attacked David Stevens on the White Rock Trail and stabbed him to death with a machete.

Today jurors saw photos of the fatal injuries suffered by Stevens — experts say he took at least a dozen blows with a machete with most of them to the head. The first witness on the stand Monday was the Dallas County Medical Examiner who said Stevens’ injuries were so severe that he wouldn’t have survived even if the attack had happened on the steps of the best hospital.

Several people on the trail the morning Stevens was killed also testified. A biker told jurors he actually rode past as the attack was happening, but kept going intent on intercepting other joggers headed in the suspect’s direction. The man said he told one young woman he encountered to “turn around and run as fast as you can.”

Prosecutors say Johnson waited under the Walnut Hill bridge, then repeatedly stabbed Stevens in an apparent random act of rage.

(credit: CBSDFW.COM)

The last witness to take the stand before lunch was a worker with Dallas Parks and Recreation. The man said Johnson flagged him down after the murder and used his cell phone to call 911. That emergency call could be played for the jury and Johnson could be heard saying, “Ya’ll need to come do something about this dead body.” and “I don’t want no police pulling up trying to kill me.”

Prosecutors told the jury that after the stabbing Johnson told responding officers he was the one who committed the murder. Dallas County District Attorney Andrea Moseley said, “We have a confession from his own mouth. We have DNA evidence [and] it will show you who committed the crime. At the end of all the evidence there should be no doubt in your mind.”

While Johnson has pleaded not guilty, his defense team isn’t disputing that he killed Stevens. Defense Attorney Paul Johnson told jurors, “I suggest to you, more importantly, that if you do not allow yourself to become so overwhelmed by the grievous injuries that occurred it will open your mind to a consideration of the evidence that you will hear as the trial concludes.”

Defense lawyers suggested that Johnson’s mental state should be at the center of the trial.

About six months after Johnson’s arrest in 2015, a judge declared him mentally incompetent to stand trial. He spent enough time in treatment and evaluation at a state hospital, though, to regain competency, enabling the trial to begin.

As a teenager Johnson was a star football player at Skyline High School in Dallas and later played for Texas A&M University. He walked away from college and it was at that point that his life seemed to begin a downward spiral.

A photo of husband and runner David Stevens who was attacked and stabbed to death as he ran near White Rock Lake. (credit: Jennifer Lindgren/CBS 11 News)

Stevens, 53, was an avid runner, out for a morning jog before going to his job at General Electric, where he worked as an engineer. Days after his death, Stevens’s wife took her own life.

In all, prosecutors are expected to call more than two-dozen witnesses to testify during the trial. The defense has indicated it will put 7 people on the stand.