Lewthwaite reportedly rented a unit at the Westgate Shopping mall months before the attack in preparation for last week's shocking massacre.
Newspapers were glued to the windows of the unit to conceal what was going on inside, as Lewthwaite pretended to be stocking up on goods.
Staff reportedly told police in Nairobi this week that they helped a woman fitting Lewthwaite's description lift boxes into the shop unit.
According to eyewitnesses a woman strongly resembling the 29-year-old mother of three was seen being led away among panicked survivors, with her face and upper clothes visibly bloodstained.
Interpol has reportedly issued a red flag arrest warrant for Lewthwaite, based on intelligence that she led the attack and managed to escape in the chaos that followed.
According to the Belfast Telegraph shocking stories about the brutalities inflicted by the terrorists are beginning to emerge. Children were reportedly among those deliberately killed and there are reports that an infant was decapitated and the head thrown at Kenyan soldiers during the four-day siege.
Lewthwaite was allegedly the leader of the 13 to 17 terrorists who carried out the attack, but managed to evade detection as the others were cornered by Kenyan soldiers.
The latest reports strongly contradict previous ones given last week that Lewthwaite was simply a book-keeper or fund raiser for the al-Qaeda-affiliated Somali group, Al Shabaab.
Kenyan police now reportedly believe Lewthwaite, who was born in Banbridge, but was brought up in England where she converted to Islam in her teens, was the leader behind the Westgate Shopping Mall attack, a prime target for the Islamist terrorists because it is Israeli-owned.
Lewthwaite has been sought by Kenyan police since she moved to Nairobi in 2011 after she reportedly spent three years living under false identities in South Africa. Police believe by this time she had already established connections with the Somali terror group Al Shabaab and married one of its senior figures. Her second husband was reportedly shot dead in a police raid in Mombasa last October.
Kenyan police named Lewthwaite as an Al-Shabaab member last June after arresting another British-born member of the group, Germaine Grant. Grant was the terror group's financier working under Lewthwaite, police believe. Lewthwaite was reportedly behind an attempt to free Grant from prison in Mombasa where he is currently on trial for terrorist offences.
Lewthwaite allegedly married another Londoner, Habib Ghani, who also joined the terror group in Somalia. But Ghani and another US-born member of the group named Omar Hammami were both shot dead during internal feuding at the start of the month.
In June this year Lewthwaite was named by Kenyan police in connection with a grenade and gun attack on the Jericho Beer Garden in Mombas, while customers were watching a Euro 2012 quarter-final match, killing three and injuring 30.
Meanwhile details of last Saturday's massacre in Nairobi are still emerging but it is expected that the official death count of 67 will rise considerably. Part of the centre collapsed after an explosion under the roof-top car park. At least 60 people are still unaccounted for.