He said that four candidates, who have not been involved in the trial in The Hague, were being considered for the job and a decision was expected "relatively soon".
"Certain witnesses who appeared after this trial opened, when testifying before the chamber, referred to pressure or to attempts to intimidate to which they were subjected by investigators for the prosecution as well as to irregularities during their preliminary interviews," documents released on Wednesday said.
One witness said he and his family were subjected to "tremendous pressure" to give a statement, including 30 to 50 telephone calls a day.
Some said the contents of their statements did not match their words, another was allegedly threatened by investigators that they would "continue questioning him until he signed".
One witness said "the prosecution had told him that if he testified, after that he could go to America, that he would get a good salary and would get money".
Another alleged he was poisoned.
"The chamber obviously took these allegations very seriously and decided that it would be wrong to leave any space for doubt arising either on the protection of the rights of the accused or on the investigation techniques by members of the OTP," Chartier said.
Seselj is standing trial for his alleged role in the persecution of Croat, Muslim and other non-Serbs and their expulsion from areas of Croatia, Bosnia and Serbia between 1991 and 1993.
He has himself been the subject of two procedures for contempt of the court for allegedly revealing the names of protected witnesses.