"The commission wishes to request for Certified Copies of Declaration of Income, Assets and Liabilities forms for years 2012 to 2022, including the initial and final declarations," reads Mbarak's letter.
Mbarak said the Anti-corruption body was carrying out investigations on Dr Matiang'i.
"The commission is carrying out investigations involving Dr Fred Matiang'i, former Cabinet Secretary for Interior and Coordination of National Government," said Mbarak.
The EACC boss said their officers Mr Abdul Low and Mr Paul Mugwe will be available to collect the documents.
During the 10 years of former President Uhuru Kenyatta's two terms, Matiang'i served as CS for Information and Communications in 2013. He was later moved to Education, Science, and Technology in December 2015 having previously been in an acting capacity as the Cabinet Secretary for Lands.
He was later transferred to the Interior Ministry in 2017 and elevated a year later to chair of Cabinet sub-committee meetings.
According to section 30 of the Public Officer Ethics Act, the contents of a declaration or clarification under this Act shall be accessible to any person upon application to the responsible Commission in the prescribed manner if the applicant shows to the satisfaction of the responsible Commission that he or she has a legitimate interest.
It noted that; "No information obtained pursuant to subsection (1) shall be published or in any way made public except with the prior written authority of the responsible Commission." As the EACC was writing to him, another letter from the Immigration Department to the Director of Criminal Investigations (DCI) dated February 28, 2023, revealed details of when Matiang'i left the country and the triangulation of his travels in the recent past.
An official at the ministry confirmed authenticity of the letter that indicated Matiang'i left the country a week before he was summoned.
Last week, Matiang'i was summoned by the DCI for investigation over what they alleged to be a possible fake raid at his Karen home.
In a letter, detective Michael Sang directed Matiang'i to appear before him at Mazingira House at 9.30am without fail.
However, Matiang'i through his lawyers responded that he will not be available for grilling by the detectives given he was out of the country.
Sang's letter had asked Matiang'i to appear before him at Mazingira House at 9.30 am without fail.
Sang said; "I have reasons to believe that you, Dr Fred Matiang'i, the former Cabinet Secretary, Ministry of Interior and National Co-ordination, is connected to the offence or have information which can assist me in my investigations."
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The detective's letter follows a purported and widely reported raid at the CS's home which the police denied never happened and sought CCTV footage from Matiang'i's home to verify the veracity of the claims.
A few days later security officers forcibly entered the residence on February 15 and ransacked the house.