Witness statements uncovered by a team investigating al-Shabaab terrorist Evans refer to a mystery woman who was involved in the atrocity that claimed the lives of more than 50 people in the Kenyan countryside in June last year.
A man who identified Evans as one of the killers also saw “a white woman with a skirt, holding a big gun and with a chain of bullets around her chest”.
A security source said: "There was another credible sighting of Lewthwaite in Lamu County on the Kenya-Somalia border, very close to this massacre, earlier in 2014.
"We know she has been closely involved with al-Shabaab in this region.
"Another relevant factor is this woman tried to cross into Somalia very close to where Evans and his cohorts were gathering.
"She disappeared after being challenged by immigration officials."
Mum-of-four Lewthwaite , 31 – who has been dubbed the world’s most wanted woman – is not named in the dossier.
But the previous apparent sighting in the same border region suggests she is the female terrorist that was spotted during al-Shabaab’s mass killing in the village of Mpeketoni.
Lewthwaite and Evans are from towns 15 miles apart in Buckinghamshire.
In the African border villages where Evans slaughtered innocent civilians he became known as the White Beast.
The former electrician, from High Wycombe, who changed his name to Abdul Hakim, was killed aged 25 when his 50-strong unit crossed back from Somalia into Kenya in June this year.
They were shot down by Kenyan troops waiting in ambush on the anniversary of the Mpeketoni massacre.
An image of Evans’s bloodied corpse was posted on Twitter. His devastated mum Sally Evans had not seen her son in four years.
Investigators on a Channel 4 documentary called My Son the Jihadi, due to be broadcast on Thursday, gathered the gruesome evidence of Evans’s atrocities.
Speaking for the first time about the dossier, Sally, 57, said: “I can’t believe that he did that and he thought he was doing the right thing. It sickens me to think he did it. I’m so ashamed of him.
“It’s easy for me to remember him as the Thomas I knew, but I know I have to face up to what he’s done.
“I have to accept what he is capable of. I can’t work out how he could justify it.”
She added: “I suppose you put two and two together and think of Lewthwaite, but Thomas never mentioned her.”
The witness statements relate to two al-Shabaab massacres in Lamu County in which more than 90 people were butchered in total. Footage filmed the night before the Mpeketoni slaying in June 2014 shows bearded Evans smiling as he hugs fellow fighters.
Ann Wairimu was staying in a guesthouse with her husband when the terrorists shot their way in then marched the residents into the street at gunpoint.
She looked on in horror as her husband was murdered.
“When we got outside we saw one white man,” she said.
“He was smiling at us and telling us, ‘Now we’ve got your men. We will kill them in pain while you women watch them dying’.
“They put our husbands on the ground and forced us to watch them being slaughtered. The white man removed his big knife from his chest and started to slaughter them one by one, and then he removed a camera and filmed them being shot by the other attackers.”
Another Kenyan, Samuel Njuguna, told how he and four friends were surrounded and told to lie on the ground. Evans then interrogated them.
“A white man comes near us and starts to ask our names,” he told investigators. “My friend was asked, ‘What’s your name?’ in English by the white man. My friend answered Kamau and the white man cut his head and other attackers shot him.
“My second friend was asked and answered Peter. He was shot in his chest and the white man cut his neck. My third friend’s name was Komu. They killed him.”
Samuel said that an attacker asked him his name while Evans cleaned blood from his knife.
To save his life, Samuel claimed he was called Omar, an Islamic name.
Evans and his crew then murdered Samuel’s only remaining friend.
Motorbike repairman Mwangi Maina identified the white woman, potentially Lewthwaite, at the scene.
The second attack, in nearby Hindi, was carried out in July, 2014.
Again men were targeted.
Mum-of-eight Miriam Mwenje, 56, recalled Evans pushing his victims on the ground with his gun and ordering them to stay down. He slaughtered one man from behind with his knife.
Another resident, Elizabeth Odipo, 62, said Evans declared: “They have killed our people in Somalia and taken this land. All of you move away; this here is Somali land. When we come back, we will kill every one, even chickens. For now we are only killing men and young boys.
“When we come back again we want to find you have converted to Islam.”
Evans and the other thugs later set fire to the house with Elizabeth’s children still inside.
Reading through the report with Evans’s younger brother Michael, 23, at her side has been a harrowing experience for former teaching assistant Sally.
She is struggling to reconcile the monster described by Kenyan villagers with her beloved little boy, who travelled to Somalia via Egypt in 2011.
For nearly four years he roamed through the Somalia’s lawless badlands with a ragtag bunch of fanatics, occasionally phoning his family in Britain for a chat.
Shaking her head in sorrow as she spoke for the first time about her son’s executions, Sally said: “It’s quite horrific. When I saw the pictures of him with his knife I hadn’t seen Thomas for four years. It was weird seeing him again.
“It was strange because he was kind of as I remembered him...
“Physically he looked like my son, but what he was doing, that wasn’t my son. It’s not the son I had brought up.
“I don’t know how he became that person and how he thought what he was doing was the right thing to do.”
Sally’s sporadic contact with Evans was interrupted when he went on the al-Shabaab mission into Kenya.
He refused to divulge what he was really doing, referring to his terror assaults as “just guarding things”.
When worried Sally voiced her fears, Evans would reassure her that if he were “made a martyr”, she would receive a call from an English-speaking American Muslim.
She never did.
“He told me they used to walk everywhere,” Sally added. “He used to tell me they would go hunting... I never knew he was killing people too.
“He used to say the people in Somalia welcomed al-Shabaab and gave them food and a place to stay. I guess they didn’t have a lot of choice.
“I did ask him, ‘Have you killed anybody?’. He never said ‘yes’ or ‘no’, he left it hanging and changed the topic.
“When he called me in August 2014 he sounded really tired. It was like the spark had gone out of him. That softness had gone.”
She added: “How do you go from being a white lad with a normal upbringing in High Wycombe to carrying out that?”
Lewthwaite, of Aylesbury, became known as the White Widow because her husband was one of the London 7/7 bombers in 2005.
She is accused of being an al-Shabaab mastermind.
Lewthwaite has been linked to the Kenyan shopping mall atrocity in Nairobi in 2013 in which more than 60 people died.
Interpol issued an arrest warrant for her in the same year.
Authorities believe she is still in East Africa.