Ukraine's Prosecutor General Svyatoslav Piskun says investigators know who ordered the murder of journalist Georgiy Gongadze in 2000.
Gongadze was editor-in-chief of an independent news website
The authorities have already announced the arrest of Gongadze's "killers".
Mr Piskun told Ukrainian television Gongadze was strangled by employees of the Interior Ministry, but he would not reveal who was behind the plot.
President Viktor Yushchenko said it was a matter of honour to solve the crime - widely seen as politically-motivated.
"The killers executed a criminal order. We know the person who gave this order," Mr Piskun said.
He said two of the alleged killers who had been detained were policemen working for the criminal investigations directorate, a department of the Interior Ministry.
Earlier media reports indicated there were four "key witnesses" in the case, but one of them had already been killed.
According to these reports, they were members of a criminal gang largely comprised of police officers.
Gongadze, who led investigations into high-profile corruption cases, was abducted in Kiev in September 2000.
His decapitated body was later found in a forest.
Gongadze's killing prompted months of protests against the then president, Leonid Kuchma, who consistently denied opposition allegations that he was involved.
A proper investigation into this crime was among the main demands of the pro-Western opposition, which staged the "orange revolution" that brought to power Mr Yushchenko following the disputed presidential elections in November.