A bullet fired by members of the Tennessee National Guard entered the back of 20-year-old Tyrin Johnson, pierced his lung and killed him, an autopsy report shows, contradicting the initial account given by officials.

Authorities previously said that Johnson was shot by two guard members in the chest on 5 July in Memphis, after he faced them with a gun.

A copy of the autopsy seen by the BBC said that a single bullet penetrated his back on the right side before exiting through his upper right chest.

Johnson's family is calling for accountability following the release of the autopsy report. The National Guard did not immediately respond to the BBC's request for comment.

"Be transparent - you shot him in the back," Evaniel Johnson, the slain 20-year-old's grandfather, told the Associated Press.

"I just want to make sure somebody be responsible for what happened. Some accountability. They ain't never going to bring my grandson back.

"If he ain't caused you no threat, you know what I'm saying, you have no reason to be killing," Johnson, a former Nashville correctional officer, added.

The autopsy was conducted by the West Tennessee Regional Forensic Center Office of the Medical Examiner.

Its report included the "investigative narrative" of an agent with the Tennessee Bureau of Investigation who said Memphis police and National Guard troops were first called to scene after reports of nearby gunfire.

They encountered Johnson, the investigative narrative said, who then "grabbed at his waistband".

"Law enforcement officers saw the unknown male draw a firearm," according to the officer's account of events, included in the report. "Law enforcement officers then shot at the unknown male, striking him once in the chest."

Separate from the officer's account, the report also said Johnson was carrying a "handgun magazine with rounds".

In Tennessee, gun laws allow legal gun owners to openly carry weapons.

"The autopsy findings reinforce the serious concerns we have regarding Tyrin's shooting and the circumstances surrounding his encounter with the Memphis Safe Task Force," the Johnson family's lawyer Andre Wharton, who shared the autopsy report with the BBC, said in a statement.

"The fact that Tyrin was actually shot in the back, contrasted with officer and task force's accounts, only further underscores the devastating nature of this encounter."

Johnson's cause of death was listed as "homicide", the official term used when a death is caused by another person. The term does not always include intent, and no criminal charges have been filed to date.

Asked by the BBC about the conflicting findings, the Tennessee Bureau of Investigation referred to its original July statement, saying the investigation remained "active and ongoing".

"Reports that Tyrin Johnson turned and pointed a gun at National Guardsmen were not distributed by the TBI," the agency said.

Tennessee State Representative Justin Pearson released a statement on X on Friday saying "Johnson should be alive today" and called for an investigation.

"The National Guard is not welcome here," he continued. "Mr Johnson's murder is a horrifying example of the brutality and injustice the Memphis Unsafe Task Force has brought to our city."

The troops were in Memphis as part of the Memphis Safe Task Force, created by US President Donald Trump amid a larger effort to deploy National Guard troops to US cities.

Trump has said the troops deployment - largely in Democrat-led cities, including Chicago, Los Angeles and Washington DC - was meant to combat crime.