PETRINJA, Croatia (AP) _ Mijo Mladjenovic knew exactly what he was looking for and where to find it. In 1991, as a prisoner of Croatian Serbs, he was forced to collect and bury the bodies of those killed as Serbs took this town.

On Wednesday he was back in Petrinja, showing Croatian officials and human rights monitors where bodies were buried.

Mladjenovic and his partner in the grisly work four years ago, Mladen Korecic, are among a cadre of survivors providing evidence of the murderous history of the wars in Croatia and Bosnia. The rapid change of front lines in recent months has allowed mass graves in both republics to be unearthed.

His bright blue eyes blazing, Mladjenovic carefully watched excavation work and tried hard to remember.

``This is Visnja Kosanovic, I'm sure!'' he shouted as experts exhumed a 17th body, that of a woman, from 60-foot-wide pit. The body still had traces of underwear stuck to it.

``I remember that she wore a women's slip and was barefoot when we put her corpse into the truck,'' he said.

Wet clay earth in a uncultivated field inside the former Yugoslav People's Army compound in Petrinja preserved the bodies.

Hair remained on some of the bodies. There were remnants of clothes and teeth, a gold bracelet, a watch stopped at 7 p.m. Flies and stench were inescapable.

Thirteen corpses were found on the first level of the mass grave. More were below. As evening fell, experts were discovering a third level. European Union monitors watched the work. Investigators from The Hague war crimes tribunal were in Petrinja on Tuesday.

Mladjenovic, a plainly dressed 46-year-old factory worker, and his friend Korecic, 36, a one-time Petrinja policeman, testified that 30 to 40 people were buried here after Serbs took Petrinja in the Serb-Croat war in 1991. Petrinja, about 30 miles southeast of Zagreb, fell that September.

The two are the only survivors of a five-member work brigade ordered by Serbs to collect and bury bodies. It took them four days. Bodies were buried at two sites in Petrinja.

``The bodies were all of elderly people who did not manage to flee,'' Mladjenovic said. ``We were finding them killed in houses, basements, backyards _ some by pistols, some by knives. We found one with the head cut by an ax.''

``Some of them were our friends,'' he whispered.

Seventeen corpses of Croatian soldiers already have been excavated from another grave in Petrinja.

``As they were throwing the corpses down from the dump truck to a pit, I thought that as we finish with them, we will end up there, too, because we were witnesses,'' Mladjenovic said. ``Luckily, we survived.''

Croatian authorities claim to have located up to 50 mass graves. Maj. Ivan Grujic, head of Croatia's commission for missing persons, said the excavations may help solve the cases of 2,846 people still regarded as missing from the 1991 war.

Mladjenovic and Korecic spent about 15 days in prison in Petrinja, then were transferred to a prison in nearby Glina.

``It was good when they beat us with police truncheons. Harder, for some fatal, was when they used chairs, cables, or when they connected people to electricity,'' Korecic said. ``Some of these Serbs were our neighbors.''

Korecic finally was released on Oct. 30, 1991, Mladjenovic in March 1992. Petrinja remained in Serb hands until this August, when Croatia recaptured large pieces of territory.

Serbs held about one-third of Croatia after the 1991 war, in which at least 10,000 people were killed. Now they hold less than 5 percent, all in eastern Croatia near the border with Serbia.

When Croats recaptured Serb-held land, they, too were accused of atrocities, burning and looting. Most of the Serb population fled to Bosnia and Serb-led Yugoslavia. Four suspected mass graves have been found.

Korecic and Mladjenovic said they would seek no revenge against Serbs, but that they would also never forget.

All the time he was held by the Serbs, Mladjenovic said, ``I feared for the worst, that they would kill us. But I also thought: they cannot kill us all. At least one will survive. To tell.''