Two witnesses in the International Criminal Court case against Kenyan president Uhuru Kenyatta have withdrawn their testimony, according to a court document.
At least one witness said he would be in danger if he testified, according to the document published on Thursday.
The withdrawal of the two witnesses, who were not named in a redacted public version of the filing, is another blow to prosecutors.
They are struggling to pin charges of crimes against humanity on Kenyatta and his deputy William Ruto, who they say orchestrated post-election violence five years ago in which 1,200 people were killed.
Prosecutors were forced to drop their case against Kenyatta's co-accused, civil servant Francis Muthaura, in March, with the court's chief prosecutor Fatou Bensouda saying witnesses were either dead or too afraid to testify.
"The case against Kenyatta has already been very shaky since the witness withdrew in the Muthaura case, so the question is whether this will be enough to push it over the edge," said William Schabas, a law professor at Britain's Middlesex University.
In their filing, dated 16 July but published two days later, prosecutors say one witness, known only as Witness 5, believed he faced an "insurmountable" security risk.
"It appears Witness 5's concerns for his security...have become too great for him to bear, and he has decided to withdraw in consequence," prosecutors wrote.
The reasons for the withdrawal of another witness, number 426, were redacted in full in the published filing, but prosecutors said all attempts to "mitigate his concerns and secure his attendance at trial" had been unsuccessful.
Kenyatta and Ruto, who were on opposite sides of the clashes in 2008, were brought together by the ICC charges both faced, winning presidential elections on a joint ticket in March.
The ICC charges played a central role in the campaign, prompting accusations that the court was disrupting the political process in a country where post-colonial sensitivities run deep.