Omar Khadr, the Canadian who was once the youngest prisoner at Guantánamo Bay, is unsure about his memory of a firefight that led to his murder conviction by a US military tribunal, according to an interview published on Wednesday.

Khadr, 28, was the first person since the second world war prosecuted in a war crimes tribunal for acts committed as a juvenile.

In 2010 he pleaded guilty to charges that included murdering US army medic Christopher Speer with a grenade in a firefight in Afghanistan in 2002, when Khadr was 15.

He has recanted that confession since returning to Canada in 2012 and was released on bail this month while he appeals against his conviction.

In an interview with a Toronto Star journalist and documentary film crew, Khadr said he agreed to the deal because he was advised it was his only way out of the US naval base prison on Cuba.

“I have memories but I don’t know if they’re mine, if they are accurate or not,” Khadr said, adding that he was unconscious for a week after the event and then brutally interrogated.

Khadr was blinded in one eye and shot in the back in the battle. His lawyers have argued there is evidence he did not throw the grenade that killed Speer.

Khadr said his vision and memories were foggy after he was initially injured in the firefight but he recalled being given a grenade.

“I got scared. I was thinking ‘What should I do?’ I didn’t know what to do, so I thought I’m just going to throw this grenade and maybe scare them away … It was the only thing I had and I didn’t know what to do, so I lobbed the grenade behind me,” he said.

“For the longest time I thought that’s what happened; whether it did or not, I don’t know. I always hold to the hope that, you know, maybe my memories were not true.”

Khadr’s case has divided Canadians. While the government opposed his release, human rights advocates such as Amnesty International say the one-time child soldier has been denied due process.

Khadr was taken to Afghanistan by his father, a senior al-Qaida member. He apprenticed the boy to a group of bomb makers who opened fire when US troops went to their compound.