Chris Lane was not shot in the back and left to die on the side of a road in Oklahoma because he was white or the victim of a gang initiation, the lead prosecutor and police chief investigating the Australian's murder believes.

Inflammatory, race-based comments on Twitter by the youngest accused, 15-year-old James "Bug" Edwards, before last week's drive-by murder in the city of Duncan led to speculation Lane, 22, from Melbourne, was targeted because of his race.

In one Twitter post Edwards wrote: "90% of white ppl are nasty. #HATE THEM".

In another post Edwards, a champion junior wrestler who hoped to represent the US at the Olympics, claimed to have knocked five white people out after July's controversial acquittal in Florida of neighbourhood watch co-ordinator George Zimmerman for the second-degree murder of 17-year-old Trayvon Martin.

"Ayeee I knocced out 5 woods since Zimmerman court!:)," Edwards posted, using the derogatory word "woods" to describe whites.

Stephens County district attorney Jason Hicks said there was not enough evidence collected so far in the investigation to indicate Lane's killing was motivated by race.

"At this point, the evidence does not support the theory that Christopher Lane was targeted based upon his race or nationality," Hicks said in a statement.

"The evidence is insufficient to establish that race was the primary motive in the murder of Christopher Lane."

Edwards and 16-year-old Chancey Luna have been charged with first-degree murder and face the prospect of life sentences without parole while 17-year-old Michael Jones, the alleged driver of the drive-by vehicle, has been charged with being an accessory to murder and faces a potential 45-year sentence.

Hicks noted Luna's mother and Jones were both white.

Duncan police chief Danny Ford has also hosed down speculation the three boys were members of a gang and the targeting of Lane as he jogged along a road was part of a gang initiation.

Ford believes it is possible the boys were "wannabe gangsters".

"It's the idea that, 'I'm a gangster, you need to respect me, you need to give me attention, you need to be afraid of me'," Ford told the Duncan Banner newspaper.

"The problem is when you market yourself, someone eventually begins to say, 'Well, OK, if you're really going to market yourself that way then demonstrate to us', and they feel like if it got to that situation they had to demonstrate to maintain the status they were trying to get."

Ford said one of the boys had confessed that Lane, 22, was shot for "the fun of it" and because they were "bored".

Duncan residents, who have been rocked by the murder, held a service on Friday evening at a school to honour Lane.

More than half the 3763 students at Duncan's public schools stayed away from classes on Wednesday after local police received "anonymous threats" involving Duncan High School, while a local gun shop reported a surge in residents buying hand guns to protect themselves.

An online memorial fund set up by one of Lane's US baseball team-mates has raised a staggering $155,000 ($A173,000) in just three days.

It was hoped the fund would raise $US15,000 to help pay for funeral expenses and take Lane's body back to Melbourne. But the outpouring of support from people around the world will allow Lane's parents to set up a foundation to make donations to organisations Lane was passionate about.