Private had been in East Timor less than 48 hours when his platoon found Sander Thoenes.
The 30-year-old Dutch journalist, working for the Financial Times, had disappeared off a stretch of road leading to Becora, a Dili suburb. Stewart was among the first troops to see his badly mutilated body, dumped in backyard of a house.
These sights haunted the 23-year-old.
“In Dili there were dead bodies in the gutters, limbs, people who had been scalped,” a former army colleague in 2005. “At the end of the day we received a grand total of zero counselling.”
Stewart, an infantryman from Mooloolaba in south-east , returned from his six-month tour of duty shattered.
In early 2001, at Brisbane’s Enoggera army barracks, he was reportedly treated for post-traumatic stress disorder, diagnosed unfit for service and discharged.
Fourteen years later, the suspicions of intelligence agencies about what happened next appear to have been confirmed: shortly after he was discharged, Stewart plunged himself back into combat, joining the Taliban in Afghanistan in late 2001, taking up arms with al-Qaida and eventually resurfacing as a trainer in its .
Stewart’s parents, Vicki and Peter, had written to the ADF asking for a report of their son’s condition and guidance on how to help him. They apparently lost touch with their son when he boarded a flight for Kuala Lumpur on 4 August 2001. He told them it was a holiday, that he needed a break. They have never spoken again.
Department of Foreign Affairs records show Stewart crossed the border into the same month, according to the book Australian Jihad, by Martin Chulov, now the Guardian’s Middle East correspondent.
By March 2002 suggestions were aired Stewart was inside the war theatre, after his name reportedly appeared in documents seized by US troops from an al-Qaida training camp.
His family shrugged off the claims, even after the defence force admitted a former soldier fitting his description had joined al-Qaida.
In 2004 Vicki and Peter held a small, private wake for their son, believing the former Immanuel Lutheran College student had died somewhere along his travels. They maintained that belief even when a gun-wielding, balaclava-clad militant with an apparent Australian accent surfaced in an August 2005 al-Qaida propaganda tape.
“As we are killed, you will be killed. As you bomb us, you too will be bombed,” the dark-eyed figure told the camera.
By then, the government was admitting Stewart was one of a number of Australians .
According to Chulov’s account, when next arrived to question Vicki, they played her the video and asked whether she recognised the voice. She said she did not. The family later released a statement insisting the man in the video was not Mathew, Chulov wrote.
He quoted a senior security official in early 2006, admitting voice analysis of the tape was inconclusive. “It is an open question,” the official said. “But we have good reason to believe he is alive.”
Stewart’s story resurfaced at the weekend in Jabhat al-Nusra propaganda, filling in some of the blanks of his mysterious past. According to his unverified account, he converted to Islam in Afghanistan around August 2001.
(He fudges the month, suggesting it could also have been July that year, when records show he was still in Australia.)
He claims he fought in Kandahar and then Tora Bora, before retreating in 2003 to Waziristan, the lawless mountain region between Pakistan and Afghanistan.
There he said he “conducted operations” against Afghan and Nato forces, and met senior al-Qaida figures, including Abu Yahya al Libi, a rising star in al-Qaida until he was killed by a drone strike in 2012.
Stewart now claims to be in , where he is assisting al-Qaida’s Syrian branch, Jabhat al-Nusra, as a “military trainer and advisor”.
On Tuesday, Vicki Stewart, working at her realty firm on the Sunshine Coast, did not respond to approaches for comment. A solitary car sat in the driveway of another family member in Mooloolaba, who later abruptly ended a phone call with Guardian Australia.
The AFP said it did not comment on security or intelligence matters.