Police have arrested four suspects in an investigation into the deaths of 18 elephants in Kenya's Amboseli ecosystem, where authorities also seized assorted substances including chemicals, the tourism and wildlife ministry said.

A Kenya Wildlife Service (KWS) spokesperson told Reuters on Monday that the four have since been released on bail.

Preliminary laboratory results point to the probable involvement of a toxic substance, the ministry said in a statement released on Saturday, with earlier indications suggesting cyanide poisoning due to the ingestion of pesticide-contaminated tomatoes from farms bordering Amboseli National Park.

Some wildlife experts have expressed doubt that farm pesticide was responsible for the deaths, citing the absence of reported deaths among other animals in the area.

The ministry said investigators were also pursuing a fifth individual believed to have information relevant to the case. A search in Tikondo in Loitoktok sub-county, about 220 km (137 miles) southeast of Nairobi, led to the discovery of assorted substances, including chemicals believed to have come from a neighbouring country, the statement said.

The ministry said the arrests and recovered substances did not in themselves establish criminal culpability and added that investigations were ongoing.

The first 15 elephant deaths were recorded between June 24 and July 24, the ministry said. In July, KWS said the deaths occurred in Amboseli National Park, Kimana Sanctuary and the Kuku Ranch area, and that the animals showed symptoms including partial paralysis before dying within one or two days.

The broader Amboseli ecosystem spans the national park and surrounding community land in southern Kenya near the Tanzanian border, providing habitat and movement routes for elephants and other wildlife beneath Mount Kilimanjaro. 

The area has a history of human-wildlife conflict, with elephants roaming beyond protected areas into community and agricultural land, trampling crops and angering farmers.