Four former Serbian secret service officials have pleaded not guilty in the 1999 killing of a prominent journalist during late leader Slobodan Milosevic's autocratic rule.

Prosecutors said Monday at the trial's opening that ex-spy agency chief Radomir Markovic and three others planned the killing of journalist Slavko Curuvija outside his apartment in Belgrade. The killing occurred during NATO's 1999 air war against Serbia to halt its attacks on people in Kosovo.

Curuvija was a critic of Milosevic. Days before he was gunned down, he was singled out by Milosevic's powerful wife, Mirjana Markovic, as a supporter of the NATO bombing.

Curuvija's family has accused Markovic of ordering the attack. She has been granted political asylum in Russia.

Milosevic died in 2006 while on trial at a UN war crimes court.