Kenya's director of public prosecutions has ordered an investigation into the killing of a man who was a defense witness in the case involving the country's deputy president William Ruto at the International Criminal Court (ICC).

Ruto is facing charges of fomenting ethnic violence after Kenya's 2007 election. Prosecutors dropped similar charges against President Uhuru Kenyatta last month.

Meshack Yebei was described by Ruto's lawyer Karim Khan as a "critical witness to the defense."

The ICC said it had offered Yebei security measures, including safe residency in a new location, but added that he had returned to Eldoret, the region in western Kenya where he was abducted on December 28 and found dead five days later.

Yebei's role in Ruto's planned defense was not widely known before his death.

Ken Wafula, a human rights activist in contact with Yebei's family, claims that Yebei was a prosecution witness, but he had been dropped. Wafula also claims that Yebei had been offered bribes to stop being a prosecution witness. He believes Yebei's death was premeditated.

"The killing to me was planned but I would not want to say that the deputy president has anything to do with it," Wafula told DW.

Wafula also called for an investigation into "the people responsible for the terms and conditions of the witnesses recanting their statements."

Adan Duale, a senior member of Ruto's United Republican Party, has dismissed Wafula's claims.

Local media reported that after Yebei's disappearance, his mother received a text message purporting to be from her son saying he was safe in Uganda with ICC officials.

But the ICC said in its statement it had not been touch with Yebei at the time of his abduction.

The ICC's chief prosecutor Fatou Bensouda dropped charges against Kenyatta due to a lack of sufficient evidence, which she attributed to intimation of witnesses and the Kenyan government blocking her investigation.

The prosecution in Ruto's trial had also made accusations of bribery and witness intimidation.