The quartet, a chief inspector and three corporals were arrested in connection with the July 25 disappearance of two Indians, Mohamed Kidwai, Zulfiqar Khan and their driver Nicodemus Mwange in Nairobi.
The three were kidnapped on July 25, 2022.
The Indian citizens are believed to have been invited into the country to help the UDA party with election preparedness but went missing under unclear circumstances.
More than 100 police officers were deployed to the Aberdare Forest to search for Mohamed Zaid Sami Kidwai, Zulfiqar Ahmad Khan and their driver, Nicodemus Mwania Mwange.
The SSU officers suspected to be linked to the disappearance and death of the three appeared before the Internal Affairs Unit detectives who are handling the case.
Ten officers had appeared before the unit for grilling on Monday and Tuesday, but more of their colleagues were added to the list of officers under investigation.
Those summoned include two Chief Inspectors, an inspector, three sergeants, nine corporals and five constables. Yesterday, President Ruto revisited the issue of the SSU, saying no police officers should be involved in the death or forced disappearance of suspects.
He challenged the police to ensure public safety and security and to carry out their work without killing suspects.