Self-confessed Islamist militant Omar Khadr cheered himself up during his time in custody by remembering how he killed a US soldier with a grenade, a military tribunal has heard.
Khadr also told US interrogators where to find bombs planted in Afghanistan.
The Canadian, who pleaded guilty at Guantanamo Bay to war crimes charges on Monday, is awaiting sentencing
Caught in 2002 at the age of 15, he is the fifth Guantanamo inmate to be convicted at the military tribunals.
In a statement agreed by both sides at the tribunal, Khadr admitted killing a US soldier in a 2002 grenade attack in Afghanistan.
"[While in custody] he would recall his killing of the US soldier and it would make him feel good," according to the statement, called a stipulation of guilt.
The tribunal heard how US forces were able to disarm 10 roadside bombs that Khadr told them about during interrogation.
Bin Laden link
Khadr pleaded guilty to five charges, including conspiracy with al-Qaeda terrorists and murder in violation of the laws of war.
Reports say a plea deal he made will see him avoid a life sentence, and face a maximum of eight more years in jail.
In a statement accompanying the plea, Khadr acknowledged that he was ineligible for protection as a prisoner of war under the Geneva conventions because he was not a member of an organised regular fighting force.
He admitted to training for combat, building and planting bombs, attacking US troops in a firefight at a compound in Afghanistan in July 2002, and having worked as a translator for al-Qaeda and another militant faction.
Khadr, originally from Toronto, had lived in Pakistan and Afghanistan with his father Ahmad Khadr, described by US authorities as a high-level al-Qaeda militant with links to Osama Bin Laden. Ahmad Khadr was killed in 2003 in a firefight with Pakistani troops.
"Khadr indicated that his father made statements that the training was to be used in attacks against the Jews because the Jews are always fighting," the plea document stated.
Khadr also said he had been told about a $1,500 (£950) reward for each American killed.
"Omar Khadr indicated that when he heard about the reward, he wanted to kill a lot of [Americans] to get lots of money," the plea document stated.
The document said no civilians or members of coalition forces had been injured or killed by bombs planted by Khadr and his team.
But it stated that during the fatal July 2002 firefight, Khadr could have surrendered.
Instead, he stayed to fight, throwing a grenade that killed US Army Sgt First Class Christopher Speer. Khadr was wounded by US fire and captured at the conclusion of the battle.
Khadr's lawyers said he was a child soldier forced to fight by his family. They said he was a boy intimidated by "bad men" who ordered him into battle.
The US is the first country since World War II to prosecute a person at a war crimes tribunal for actions allegedly committed as a juvenile.
A seven-member military panel is to determine Khadr's sentence.