Mercep is on trial for the unlawful arrest and torture of 52 people and the murder of 43 civilians, mainly Serbs, from the Zagreb, Kutina and Pakrac areas at the start of the war in 1991.
Seks said he knew about the Pakracka Poljana case from the media and seeing the file when he was a public prosecutor between April 10 and August 10, 1992, but prior to that he had no official or unofficial information about it.
Asked if he knew that civilians were arrested and detained in the Pakracka Poljana area, Seks said he did, finding out about it from the case file when he was a public prosecutor.
Seks said that in early August 1991 he was appointed head of the Eastern Slavonia emergency centre to which the then President, Franjo Tudjman, appointed Mercep as secretary of defence for the eastern town of Vukovar.
Seks said that what he knew about Mercep's role from the summer of 1991 to his appointment as public prosecutor came only from media reports and not from "direct political life."
He met Mercep in mid-1989 when he was "a key figure in the establishment of the HDZ (party) in Vukovar," adding that Mercep was presented in the Slavonian and Vukovar public as a significant symbol of defence.
Seks said that in the summer of 1991 Mercep was transferred to Zagreb, where he "figured" as an assistant interior minister. He did not know that Mercep was on an official trip to the US and Canada in December 1991.
Seks said he found out from the media that Mercep was in the Gospic and Pakracka Poljana area, adding that the media described him as one of the key people for the area's defence.
Seks said nobody asked him to intervene about a person arrested in the Pakracka Poljana area.
The main hearing resumes tomorrow.