The anti-graft agency has started investigations into the procedure that was used to nominate Anne Mwenda as Nairobi deputy governor.
Ms Mwenda was nominated by Governor Mike Sonko on Monday this week.
Ethics and Anti-Corruption Commission (EACC) Chief Executive Twalib Mbarak has now written to Speaker Beatrice Elachi, asking for Mr Sonko’s original letter nominating Mwenda.
“This commission requires the original letter dated January 6, 2020 from the governor to the speaker, nominating the deputy governor candidate," Mr Mbarak's letter reads.
"Your timely action will be appreciated."
The Standard has established that the probe is informed by claims the governor could have backdated the nomination letter to a date prior to his arrest and prosecution on graft charges.
Directorate of Public Prosecutions boss Noordin Haji has vowed to contest Sonko's decision to nominate Mwenda, citing violation of his bail terms.
Sonko's lawyers, however, have argued that the bail terms did not bar him from exercising his executive functions since he is still the governor.
The latest investigation adds to several others targeting top officials at City Hall.
Yesterday, a special sitting convened by Ms Elachi (pictured) discussed the leadership crisis at the county, occasioned by the graft case facing Sonko.
Elachi is also being investigated over allegations of abuse of office and misappropriation of public funds.
EACC is also investigating claims made against the County Public Service Board (CSPB), that the board has been hiring and firing officials in an unprocedural manner.
To begin with, the board has been accused of ejecting Jacob Ngwele from the county assembly clerk's office, and hiring one Monica Muthami to replace him in an acting capacity.
"The board irregularly approved the appointment of Ms Muthami, the principal clerk assistant to be the acting clerk effective October 30 2019 besides making other irregular appointments and promotions,” Mr Mbarak said in another letter to Elachi.