The trial of her alleged accomplice, fellow Briton Jermaine Grant, is ongoing in Mombasa.
"This case is just like any other case, and if the suspect cannot be found, then the case should be closed," said Mombasa chief magistrate Julius Nang'ea.
Kenya issued an arrest warrant for Lewthwaite in 2012 and then requested Interpol to issue a "Red Notice" in 2013.
On Friday, state prosecutor Nicholas Kitonga pleaded that police needed more time to find their suspect, whose whereabouts are unknown.
Lewthwaite,
31, from Aylesbury in England, is a Muslim convert who was married to
Germaine Lindsay -- one of four suicide bombers who attacked the London
transport network on July 7, 2005, killing 52 people -- leading a
British tabloid newspaper to dub her the 'White Widow' years later.