British man Thomas Evans was second in command of the al-Shabaab unit he was fighting for when killed in Kenya, Kenyan police said.
Evans, 25, from Buckinghamshire, died in a thwarted attack by the Somalian al-Qaeda linked militant group on a military base on 14 June.
Christipo Mutali, from the Kenyan police, told the BBC he saw Evans carrying a camera and recording the attack before being shot.
"He was the one carrying the video during the attack. And he was commanding 'Let's move on, we are winning men, let's go, come in, let us shoot, we are winning.'
"There were two lines. He was the one leading the front line," said Mutali.
Police said that Evans can be heard on recordings on the seized camera issuing orders to younger al-Shabaab fighters.
Militant group al-Shabaab is responsible for a series of terror attacks in Kenya, including the attack on Nairobi's Westgate Mall in 2013 in which 67 were killed, and an attack on Garissa University in April in which 150 people were killed.
Evans, from Wooburn Green, converted to Islam in 2010, and soon after allegedly became involved with Muslim radicals in High Wycombe.
He travelled to Egypt in 2011, telling his family he was going to study Arabic, from where he is believed to have travelled to Somalia to enlist with Al Shabaab, where he fought under the adopted name Abu Hakim.
Evans allegedly took part in an attack on a Christian community of Lamu and beheaded a civilian, the Daily Mail reports.
His mother, Sally Evans, learnt of his death after hearing that a photograph of his body had been posted on Twitter.
"I just went numb. I couldn't believe that was my son, my little boy, my little babe who I loved," said Sally.
This week, German police issued a $100,000 (£64,000) reward for the capture of German Andreas Martin Muller, who fights under the alias Abu Nusaybah, one of the 68 fighters believed to have been involved in the attack in which Evans was killed.