BELGRADE, Serbia (AP) — The family of three American citizens killed during fighting in Serbia in 1999 said Tuesday the Balkan country's prime minister has failed on his promises to bring the suspects to justice despite repeated pledges to do so.

The Bytyqi brothers— Illy, Mehmet and Agron — left their New York pizza business to fight with ethnic Albanian rebels against Serbia's rule in Kosovo. They were arrested at the end of the clashes when they strayed into central Serbia. Their bodies were discovered in a trash-filled mass grave in 2001.

In an email to The Associated Press, the family's lawyers said "despite repeated requests, the Bytyqi family has not been given updates on any future plans to move the case along." They said Serbian Prime Minister Aleksandar Vucic "has lied multiple times to the family of three murdered American citizens. He should be held accountable."

Asked to comment, a government official said, "we have been working on the case." He spoke on customary condition of anonymity.

Bytyqi representatives claim numerous flaws in the investigation.

"Witnesses have not been given clear signals that they will be protected. No charges have been filed. Suspected war criminals, however, are faring much better," they said.

They claimed the prime suspect in the case remains a key security adviser to Vucic and Serbian President Tomislav Nikolic and their populist Progressive Party.

Kosovo declared independence from Serbia in 2008.