Sometime later, the group announced he was dead -- but police say it was only a ruse, uncovered when he was arrested trying to re-enter the United Kingdom.
Khawaja, 27, was sentenced Friday in Woolwich Crown Court to 12 years in prison, having pleaded guilty to preparing acts of terrorism and other terror-related charges, London's Metropolitan Police said.
"Khawaja chose to become a terrorist, engaged in weapons training in a terrorist training camp and faked his own death in order to conceal his entry back into the UK," Metropolitan Police counterterrorism Cmdr. Richard Walton said Friday. "This sentence sends a powerful message to those who plan or prepare acts of terrorism overseas or here in the UK."
Authorities say Khawaja, of west London, traveled to Syria in January 2014 and joined Rayat al Tawheed, a group aligned with ISIS
, the brutal terror group that has captured parts of Syria and Iraq for what it says is its Islamic caliphate.
Eventually, he took the name Abu Daigham al-Britani and appeared in one of the group's promotional videos featuring the training camp and encouraging others to join it, police said.
In a separate video posted on social media, police say, a man with his face covered is seen with a bag of severed heads. He picks up one of the heads, according to police, and shows it to the camera.
Police say a description with the video reads: "British ISIS fighter Abu Daigham al-Britani with Government soldier's head Syria."
In May, the group sent a message on social media claiming that Abu Daigham al-Britani had been killed -- news reported at the time by the British media.
Khawaja, however, was arrested on June 3 after entering the United Kingdom at the port city of Dover, Metropolitan Police said.
"The images and video of Khawaja in Syria are horrific and deeply disturbing. He was a senior figure in Rayat al Tawheed, a group aligned to the Islamic State terrorist group that celebrated its terrorist acts through social media back to the UK," Walton said.
Arrested with Khawaja was his cousin Tahir Bhatti, 45, on suspicion of assisting an offender. Police said they searched addresses linked to both men, and found messages and photographs that showed Khawaja had engaged in terrorist training in Syria, including how to handle weapons.
Khawaja pleaded guilty in December to preparation of acts of terrorism, attending a terrorist training camp, weapons training and possession of an article for terrorist purposes.
Bhatti pleaded guilty last month to assisting an offender and was sentenced Friday to 21 months in prison, Metropolitan Police said.
A second man accused of helping Khawaja, Asim Ali, 33, of west London, was arrested in August. He was accused of, among other things, giving Khawaja money before Khawaja left for Syria.
Ali pleaded guilty in December to entering a funding arrangement for the purposes of terrorism. He was sentenced Friday to 21 months in prison, Metropolitan Police said.