LJUBLJANA (AFP) - A mass grave in Slovenia believed to hold up to 300 victims killed after World War II by the former communist regime has been discovered in the country's east, authorities said Wednesday. "We've found the mummified remains of between 200 and 300 people," Marko Strovs, head of the government's military graves department, told journalists. He added the victims appeared to be "killed with gas" since there were no visible signs of wounds from firearms. Investigators and historians on Tuesday removed concrete walls built after World War II to close off the Huda Jama cave near Lasko, some 90km east of the capital Ljubljana. They then discovered the remains. The investigation of the Huda Jama cave started last August as part of a long-running probe of over 500 suspected mass graves throughout Slovenia. They are believed to contain the remains of pro-Nazi collaborators who sought to escape from the former Yugoslavia's communist regime in 1945. Slovenia was part of Yugoslavia before becoming independent in 1991. The identities of the victims in Huda Jama remain unclear. However, accounts from local residents indicate they were pro-Nazi collaborators from Slovenia or Croatia, according to the head of the Slovenian Research Centre for National Reconciliation, Andreja Valic. "Current information, based on oral testimony, indicate that the slain people could have been Slovenian or Croatian citizens," Valic told Slovenian news agency STA. State prosecutor Barbara Brezigar also visited the site and described seeing the remains as "horrible." "It is one of the most shocking things you could see in your life," Brezigar told journalists. She said any investigation into the crimes will be difficult since most of those responsible are likely dead. In July 2007, Slovenian authorities began exhuming a mass grave in the Tezno forest, northeastern Slovenia, where another 15,000 victims slain after World War II by the former communist regime are believed buried.