Three suspects were arrested and will appear in court today. But the search operation is ongoing, as additional suspects are thought to be still at large.
The service linked the killings to the recent arrest of poaching suspects.
"We had arrested 15 poachers the previous week for killing an elephant in the same area," William Kiprono, the KWS director, was quoted as saying by the Daily Nation newspaper.
The suspects were freed on bail equivalent to £690 each.
The animals were poached in a southwestern reserve on the edge of the Tsavo national park, the country's main elephant sanctuary and home to an estimated 11,000 elephants.
The adult elephants' tusks were removed. File photo / AP
Kenya is battling an upsurge in the illegal slaughter of elephants and rhinos. Since the start of the year, 65 elephants have been lost to poaching in Kenya according to figures from the KWS, which recorded 302 slayings for the whole of 2013.
Video: Elephants outfitted with GPS collars
The wildlife service estimates Kenya's elephant population at 38,000 nationwide, and the number of rhinos at just over 1,000. The KWS has been battling accusations that poaching ringleaders have been allowed to act with impunity, with five of its officials suspended this month as part of a probe into alleged mismanagement.
Kenya has fewer than 1000 rangers to fight poaching countrywide. KWS manages about eight per cent of the country's land.