All inmates suffered from some kind of mental health problems. Some could not stop thinking about food; others became paranoid; some had delusions; some lost control of themselves. Others seemed to lose their sense of hope. Some inmates reacted by trying to write about what was happening to them. For example, Nikola Nikolić, Djuro Schwartz, and Ilija Ivanović all tried to memorize and even write about events, dates and details. This was very dangerous, since writing was punishable by death and tracking dates was hard.
Most of the executions of Jews at Jasenovac occurred before August 1942. After that, the ISC started to deport Jewish prisoners to the Auschwitz concentration camp. In general, Jews were first sent to Jasenovac from all parts of Croatia after being gathered in Zagreb, and from Bosnia and Herzegovina after being gathered in Sarajevo. Some, however, were sent directly to Jasenovac from other cities and smaller towns.
Many inmates sent to Jasenovac were scheduled to be murdered. Strong men who could do hard work, and were sentenced to less than 3 years in prison, were allowed to live. However, anyone who was sentenced to 3 years in prison or longer was immediately scheduled for execution.
The Ustaše used many different kinds of systematic extermination (killing many people at once, using a system). However, they liked using manual methods of killing - killing prisoners with their hands, using tools like knives, saws, and hammers.
The Ustaše cremated living inmates. Some were given drugs, but others were fully awake. They also cremated dead bodies.
When the Ustaše first started cremating people in January 1942, they used brick factory ovens. An engineer named Hinko Dominik Picilli made cremation much easier for them by creating seven better-working crematories.
Crematories were also placed in Gradina, across the Sava River. The State Commission says "there is no information that [the Gradina crematory] ever went into operation." Later testimony, however, says this crematory was used.
Some bodies were buried rather than cremated. Their bodies were dug up late in the war.
The Nazis had used poison gas to kill many prisoners in their concentration camps. Following this example, the Ustaše tried to use poison gas to kill inmates that arrived in Stara-Gradiska. At first, they tried to gas the women and children that arrived from camp Djakovo with gas-vans that Simo Klaić called "green Thomas". Later, they built gas chambers and used Zyklon-B and sulphur monoxide to kill prisoners.
Granik was a ramp used to unload goods from boats on the Sava River. In winter 1943-44, prisoners who did farming work had no jobs to do, since the ground was frozen. Meanwhile, large numbers of new prisoners arrived. By this time, the Axis powers were expected to lose World War II, and the Ustaše wanted to kill as many people as possible before that happened. They decided to execute people on the ramp, so that after they were killed, their bodies could be dumped into the river.
Every night for about 20 days, Ustaše officers brought in lists of prisoners who they planned to execute. They stripped, chained, and beat these prisoners. Then they took them to the Granik. There, weights were tied to them; their intestines and necks were cut; they were hit in the head; and then they were thrown into the river. Over time, the Ustaše changed this method, so that inmates were tied in pairs, back to back, and their bellies were cut open before they were tossed into the river alive.
Gradina and Ustice were villages around Jasenovac. The Ustaše chose some empty areas near these villages, and used wire to mark off an area for a massacre and mass graves. They gathered many prisoners there and killed them with knives or by smashing their skulls with hammers.
When Roma people (Gypsies) arrived in the camp, they did not undergo selection, because they were all scheduled to be killed. They were taken to Gradina. In between massacres, the men would be forced to work on the dike, and the women would be forced to work in the cornfields in Ustice. Eventually, they would all be killed. Thus Gradina and Ustice became Roma mass grave sites. The Ustaše killed more and more people Gradina, until it became the main killing-ground in Jasenovac. Grave-sites were also located in nearby Ustica and in Draksenic.
Mlaka and Jablanac were used as camps where women and children were held and forced to work. However, many women, children, and other people were killed at the Sava River bank between the two camps.
According to the State Commission, as many as 50,000 people were killed here in the winter of 1941 to 1942. Evidence says that more people were also killed here after that winter.
In the late summer of 1942, the ISC sent tens of thousands of Serbian villagers were to Jasenovac. The villagers had lived in the Kozara mountain area (in Bosnia), where ISC soldiers were fighting against the Yugoslav Partisans. The women villagers were sent to do forced labor in Germany. Children were taken from their mothers and either killed or sent to Catholic orphanages. However, most of the men were killed at Jasenovac.
On the night of August 29, 1942, the camp's guards made bets about who could kill the most prisoners. One of the guards, Petar Brzica, reportedly cut the throats of about 1,360 of the new prisoners, using a butcher knife that became known as "srbosjek" ("Serb-cutter"). Other guards who admitted taking part in the bet included Ante Zrinušić, who killed about 600 inmates, and Mile Friganović, who gave a detailed and consistent report of what happened. Friganović admitted to killing 1,100 inmates. He specifically talked about how he tortured an old man named Vukasin. He ordered the man to bless the Ustaše leader, Ante Pavelić. The old man refused, even though Friganović cut off his ears, nose, and tongue every time he refused. Eventually, he cut out the old man's eyes, tore out his heart, and cut his throat. Dr. Nikola Nikolić saw this happen.
In April 1945, as Yugoslav Partisan units neared the camp, the Ustaše guards tried to get rid of evidence of their crimes and the people who knew what they had done. They tried to kill as many prisoners as possible, as fast as they could. On April 22, 600 prisoners revolted; 520 were killed and 80 escaped. Not long after the prisoners revolted, the Ustaše abandoned the camp. However, first, they killed the prisoners who were still alive. They also blew up and destroyed the buildings, guardhouses, torture rooms, crematoriums, and other parts of the camp. When they entered the camp, the Partisans found only ruins, soot, smoke, and dead bodies.
By the end of 1945, the rest of Jasenovac was destroyed.
Historians have had difficulty figuring out exactly how many people died at Jasenovac. Today, the most common estimate is that tens of thousands of people died at the camp. Before the 1990s, the most common estimate was that hundreds of thousands had died.
These estimates are very different for many reasons. The Ustaše did not keep accurate records. Different people use different ways of estimating deaths. Sometimes, the people making estimates have political biases. In some cases, entire families were killed at the camp, leaving no one to submit their names to lists of the dead. On the other hand, the lists sometimes include the names of people who died in other places; people who survived; or people who are on more than one list.
The Yad Vashem center claims that over 500,000 Serbs were killed in the Independent State of Croatia. This includes Serbs who were killed at Jasenovac. The Simon Wiesenthal Center estimates the same.