The Anti-Terror Police Unit on Monday obtained orders to detain three more suspects linked to the Dusit terror attack.
Twenty-one people were killed and several others injured during the attack on Tuesday last week.
The suspects are terrorist Ali Salim Gichunge's mother Sakina Abdalla, Mukhtar Ali and Ethiopian Issack Hussein.
ATPU Inspector Monica Githaiga said records from the suspects' phones showed they were in constant communication with some people in Somalia.
Hussein and Ali were arrested near Jamia Mosque in Mandera town on Saturday.
Hussein is alleged to be a Qoran student, while
Ali is a Mandera resident and an Imam.
Ali is accused of communicating with one of the attackers and facilitating his movement from Mandera.
Githaiga said Ali will help investigators to narrow down on the identity and operations of the attackers.
In an affidavit, Githaiga said Sakina told investigators Gichunge was born on December 30, 1992.
She also said after her son finished his O levels at Thuura Mixed Secondary School, he started doing casual jobs.
He then travelled to Isiolo to take a computer course.
A woman identified only as Mama Sammy then employed Gichunge as a cyber cafe attendant. He later moved to Shams Cyber also in Isiolo in 2014.
Sakina said her son then left for Mombasa where he got a similar job and could not be reached phone
as it had been switched off.
She said her son called her in 2016 and said he had landed a scholarship to study Islamic education in Mogadishu, Somalia.
Sakina said that is the last time she heard of her son until the attack when she saw his pictures circulating on social media.
Githaiga said electronic gadgets found with the suspects are being examined.
She said intelligence reports show the terrorists had other associates who helped them in planning and executing the attack. Githaiga said they are looking for the key conspirators.
"The suspects have the financial muscle to interfere with the investigations, plan more attacks and also interfere witnesses. They should remain in custody pending investigations," Githaiga said.
Senior principal magistrate Martha Mutuku allowed the ATPU to hold the three for 30 days for them to complete investigations.