The investigations, which are also said to have involved the National Intelligence Service are probing the Ruaraka land saga, which involved an alleged sale of public land, and the botched Huduma Namba project where close to Sh10 billion is said to have been spent "without any form accountability".
Last week, EACC wrote to the Head of Public Service seeking to know how much Dr Matiang'i was worth when he took office as Cabinet Secretary in 2013.
In a letter dated February 27, EACC chief executive officer Twalib Mbarak said they are interested in the former CS's wealth pursuant to Section 30(1)(4)(b) of the Public Officers Ethics Act.
"The commission wishes to request certified copies of Declaration of Income, Assets and Liabilities forms for 2012 to 2022, including the initial and final declarations," reads Mbarak's letter.
He added: "The commission is carrying out investigations involving Dr Fred Matiang'i, former Cabinet Secretary for Interior and Coordination of National Government," said Mbarak.
Architects of Huduma Namba said it was meant to help the government capture data of all Kenyans in its database that would help in planning. The project had also a security aspect to it. Dr Matiang'i's Interior ministry was heavily involved in the project that was intended to register all the 46 million Kenyans at the time.
Kenyans were supposed to receive Huduma Namba cards within three months of registering. However, most Kenyans never received their cards that were to act as a new form of identification.
The government approved Sh7.5 billion for the Huduma Namba project. Later, the Interior Ministry would seek an extra Sh1 billion that would be used to pay 50,000 clerks for eight extra days they worked after the month-long exercise.
The funds would also cater for the digitization of Civil Registration Services (Birth Certificates), which was termed a prerequisite of Huduma Namba, as well as to clear pending bills associated with the same exercise.
Part of the investigations will also touch on questions regarding the legal, technical and functionality of the National Integrated Identity Management System (NIIMS), starting from procurement, according to the sources.
The investigations will also be touching on the infamous Ruaraka land saga which Kenya Kwanza honchos have always sought to pin Matiang'i on.
Matiang'i and Education PS Belio Kipsang have been indicted, by the County Public Accounts and Investments Committee (CPAIC), over the sale of the Ruaraka land, which hosts two schools, Ruaraka High and Drive Inn Primary.
In its report, CPAIC claimed businessman Francis Mburu was paid Sh1.5 billion even after Parliament had established that the land was ceded to the government years ago.
Two weeks ago, Director of Public Prosecutions (DPP) Noordin Haji said a decision will be made on whether or not to charge Matiang'i over Ruaraka land scandal as soon as he receives the file.
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"For a long time, it is the media that has been asking about the Ruaraka land saga. Once the file is ready, we will make the decision and we will inform the public. We are awaiting the finalization of the financial investigations. Once that is ready, we will make an appropriate decision," Haji said during an interview at Citizen TV.
The matter was investigated by EACC which noted that the ex-CS ignored recommendations from the quality assurance and standards assessment team, who confirmed that it was public land.
"No one is above the law; anyone can be investigated. In regards to other investigations, that question will have to be addressed to the investigating authority," Haji told the media in 2019.
Matiang'i has always maintained his innocence.