They have names like Frenki, the Tigers, and the Chetniks. They were believed to have been among the most brutal combatants in the Bosnian and Croatian wars. And now, international officials say, they are working in Kosovo.
"They are well trained, well motivated, and very, very dangerous," says one Western military official.
Most recently, paramilitaries, as they are called, are believed to have carried out the brunt of an "ethnic cleansing" campaign against ethnic Albanians in Pec, a Kosovo city of 100,000 that is now nearly deserted.
But paramilitaries are nothing new to the Balkans, and they are likely to play a greater role in the future - especially if NATO sends ground troops to Kosovo.
"If NATO is coming with ground troops, I will go and defend my country with my [Tigers]," said Zeljko Raznjatovic, a notorious figure here, in a TV interview.
NATO countries, in their second week of airstrikes against Yugoslavia, have denied any intention to expand the mission to the ground. But some have questioned whether a humanitarian catastrophe can be curbed through bombs alone. Already more than 100,000 ethnic Albanians have fled the country.
Mr. Raznjatovic, or Arkan, who started a chain of ice-cream stores in Belgrade and has been convicted of robberies in Europe, was named this week as an indicted war criminal. The Hague war-crimes tribunal secretly indicted him in 1997.
The most active paramilitaries in Kosovo are believed to be under the control of Frano "Frenki" Simatovic, a former state security worker who first organized Serbian fighting units in Knin, Croatia, in 1990.
Well-armed and masked
Mr. Simatovic is believed to have about 400 to 500 fighters, who are well armed, masked, and are known to drive through Kosovo in four-wheel-drive trucks without license plates. They operate in groups of 15 to 25 men, according to Western officials, and have been seen in Kosovo since this summer. Their identities and origins are for the most part unknown, even to local Serbs.
"We don't know who they are or where they come from," says Natasa Kantic, the executive director of the Humanitarian Law Center in Belgrade and one of the few who has visited Kosovo since the airstrikes began. "We are [saying this] based on experience in Bosnia and Croatia. In reality there is little difference between [paramilitaries] and the police. The two are linked together."
International officials say the paramilitaries are being used first in Kosovo because they are better trained than the Serbian police in the region. Second, they are likely to be more difficult to prosecute for war crimes because records and evidence on them are harder to track down. If convicted, they will be harder to link to top officials in Belgrade.
"Paramilitaries acting under any authority [of the Yugoslav government] would be considered the same as the Army," says Graham Blewitt, a prosecutor for The Hague war-crimes tribunal. "But it is difficult to prove they are under the control of the Army."
After accusations of mass killings, "genocide," and ethnic cleansing, war-crimes prosecutions have become a constant source of speculation. Yugoslavia has refused to comply with the war-crimes tribunal, saying extradition is not allowed by its federal law.
Irregular fighters have a long history in this volatile land, dating back to the Hajduks, rebels who fought against the Ottoman Turk occupiers for some 400 years. Chetnik units were formed in World War I, to fight against the Austrians and Germans when the official Serbian Army retreated to Greece. The Partisans, guerrillas led by Josip Broz Tito, fought the Nazis in World War II and went on to form Yugoslavia. In Bosnia and Croatia, there were several paramilitary groups, many of which were sponsored by political parties.
Vojislav Seselj, the Serbian vice premier, led paramilitary troops in Croatia and Bosnia, but he is thought not to be involved in Kosovo.
The ethnic Albanians of Kosovo also have an irregular fighting force - the Kosovo Liberation Army, which is now engaged with the Serbian forces.
One former paramilitary from the war in Croatia is Nenad Canak, a liberal-minded politician from the northern city of Novi Sad. He says he was forced to join an irregular group as a means to make him "disappear." He survived.
Now, Mr. Canak worries, a ground war in Kosovo would mean more paramilitaries, and it could allow the Yugoslav government to get rid of its enemies by sending them on suicide missions to the front. "This is an open chance for Milosevic to draft and send opponents out to get killed," Canak says. "We already saw that in Croatia."