One thing is not a mystery in the stabbing death of Sean Schellenger near Rittenhouse Square last year: Before Michael White put a knife in his back, Schellenger, who did not have a weapon, wrapped his arms around White and tried to tackle him.

Five eyewitnesses said as much this week during the first two days of White’s trial for voluntary manslaughter. A cell phone video played in court also showed Schellenger’s take-down attempt, with White then driving his arm over Schellenger’s shoulders to stab him in the back before the two men collapse onto the ground.

Still, testimony has left one major question unanswered: What prompted Schellenger and White — who did not know each other — to fight in the first place?

Their encounter was spontaneous, witnesses have said, a random argument at 17th and Chancellor Streets. White, then 20, was working as a bicycle food courier. Schellenger, a 37-year-old real estate developer, had been out celebrating with friends at the nearby restaurant Rouge.

Witnesses have said the two men came face to face after Schellenger got out of a Mercedes-Benz driven by a friend to confront people in cars ahead of them in a traffic jam. For reasons that remain unclear, White stopped his bicycle at the intersection and put it on the ground.

No one has shed light on how or why Schellenger and White then got into an argument — or what words they exchanged.

Testimony has shown that the confrontation lasted at most a minute, and that White pulled out a knife before Schellenger charged him. No physical contact took place before then, witnesses have said, and the tackle and stabbing occurred in almost one continuous motion before Schellenger ended up face down on the ground and White ripped the knife out, then turned and ran.

“This went by so fast,” said Schellenger’s friend Uri Jacobson, who was in the Mercedes before the incident and saw it happen.

Matthew Schuyler testified that he was 20 feet away watching the incident, and described it as “a blur."

The question of what was said during the encounter could impact how jurors rule in the case. District Attorney Larry Krasner decided last week to proceed at trial on a count of voluntary manslaughter — often called a “heat of passion” killing or an unreasonable act of self-defense — instead of third-degree murder.

White’s public defenders, Jonathan Strange and Chief Defender Keir Bradford-Grey, have said it was self-defense.

During opening arguments Thursday, Strange said that White, now 22, will testify in his own defense and will say that Schellenger told him, “I’m going to beat the black off you,” before charging. He also told jurors that White was “scared for his life,” and that Schellenger — a former Pennsylvania State University football player — was drunk and aggressive.

Seeking to bolster that perception, White’s defense attorneys on Friday called Philip Birchfield to the stand. Birchfield, of Destin, Fla., testified that he encountered Schellenger in 2008 while working as a security guard at a bar in the coastal Florida city. Birchfield said other bouncers tried to eject Schellenger, but he became so unruly it took three of them to carry Schellenger outside. Birchfield said Schellenger threw punches during the encounter and eventually bit Birchfield’s elbow, leaving a scar that remains.

“He was just going wild,” Birchfield said.

Prosecutors Anthony Voci and Sherrell Dandy have sought to preempt the potential for White to say that Schellenger commented on White’s race, asking every witness who has taken the stand if they heard racial language at the corner before the confrontation. All have said no.

Bradford-Grey asked one witness, Norris Jordan — the driver of the Mercedes and a co-owner of the restaurants Lou Bird’s and the Happy Rooster — if he had used a racial slur toward any of the men in the cars in front of his in the traffic jam. A visibly upset Jordan responded that he had not.

Supporters of both White and Schellenger again packed the courtroom Friday. White has betrayed little emotion while seated alongside his lawyers.

Prosecutors rested their case Friday afternoon. White is expected to testify when the trial resumes Tuesday.