Lewthwaite, the British widow of the July 7 bomber, Germaine Lindsay, right, is already suspected of plotting to attack venues popular with Westerners in Kenya and is in hiding. She was accused of being part of a terrorist cell that allegedly involved other Britons and has been linked with the terrorist group al-Shabaab.
Concerns over her possible involvement grew last night amid reports that at least one of the attackers was a woman.
Security sources said there was no intelligence to suggest she was among the attackers but it could not be "completely discounted" yet.
Kenya has had a long struggle against terrorism because of its border with Somalia, the stronghold of al-Shabaab. The Nairobi attack is the latest in a series to involve Britons as victims or suspects. As a result, Britain and Kenya's security and intelligence services have strong links.
Lewthwaite evaded Kenyan authorities after she was accused of being involved in a cell that planned to attack a hotel or a shopping mall in 2011.