Exclusive: Channel 4 News learns the alleged leader of the Nairobi shopping mall attack was born a Christian in Kenya and is a former member of the country's special forces.
Multiple sources from within Somalia have told Channel 4 News the identity of the man who led the group behind the attack on Nairobi's Westgate shopping centre.
According to sources connected to al-Shabaab, his Jihadi fighting name is Omar or Umayr.
Omar, who is believed to have been killed in this week's siege, was a Kenyan national, born to a Christian family from Nairobi. He converted to Islam and was also a former member of Kenya's special forces.
Umayr is understood to have left Kenya for Somalia in 2005, where he joined the Kamboni militant Islamist group. Kamboni later joined with al-Shabaab.
Sources say Umayr used to teach al-Shabaab fighters martial arts and boxing at a training camp in Somalia. They say he fell out with leadership because they thought he lacked ambition.
A second man been named by Channel 4 News sources.
Known as Khadhab, he is a Somali national who worked in an Islamic bookshop in the Nairobi suburb of Eastleigh. He was arrested and imprisoned in Somalia, and it is alleged he was tortured by the CIA while in custody there.
When he was released, sources say, he crossed over into al-Shabaab territory.
The Kenyan government has not yet officially confirmed the identities of any of the attackers.
On Thursday, Interpol issued a "red notice" against Samantha Lewthwaite at the request of the Kenyan authorities. Though the red notice did not refer to the Westgate attack, it has been speculated that Lewthwaite, the wife of 7/7 London bomber Germain Lindsay, may have been involved in the attack.
A spokesman for Al-Shabaab earlier told Channel 4 News that the attack was an "offical operation", planned and carried out by al-Shabaab leadership.
Sixty-seven people are confirmed dead following the four-day siege, but that death toll is expected to rise.