Flying Squad officers on Wednesday arrested Dennis Itumbi at City Hall, Nairobi, over links to the Deputy President William Ruto "murder" letter.
Mr Itumbi is the State House digital director in the communications department.
He, however, operates from DP Ruto's office.
Officers who arrested Mr Itumbi took him to the headquarters of the Directorate of Criminal Investigations.
His lawyer Moses Chelanga later told journalists that he had not been told exactly why Mr Itumbi was arrested. He said, however, that it had to do with the letter.
Detectives at the weekend said investigations into the alleged assassination plot had narrowed down to two aides suspected to have generated and circulated the letter.
The authorities indicated that the duo from the communications department were behind the document that created a storm after four Cabinet secretaries were summoned to the Directorate of Criminal Investigation last Monday.
Once a staunch defender of the Jubilee administration during its first term, from 2013 to 2018, Mr Itumbi kept a low profile after President Uhuru Kenyatta reprimanded him and three other communication directors over the manner in which they were handling State House communication.
The others the President rebuked in June 2016 were Edward Irungu, Munyori Buku, and Eric Ng’eno.
Mr Itumbi, a bachelor and a teetotaller, came to national prominence with his virulent attacks on the International Criminal Court, which had charged Mr Kenyatta and Dr Ruto over the post-election violence that rocked Kenya in 2007/2008.
The ICC claimed Mr Itumbi hacked their systems in order to reveal the names of witnesses against the suspects.
In 2012, police arrested him over this claim and detained him for four days, but did not prefer formal charges.
He sued the government for illegal detention and was awarded Sh5 million in 2018 by Justice Joseph Onguto.