Palestinians joined the ICC with the goal of trying Israeli leaders for alleged war crimes in the fighting in and around the Gaza Strip in summer 2014, and alleged crimes relating to the occupation of the West Bank and east Jerusalem.
Qassas is one of the many Palestinians who lost family members during the July-August war, waged by Israel to end rocket attacks at its territory and destroy attack tunnels from Gaza.
Approximately 2,200 Palestinians were killed, of them 1,500 civilians, according to a recent United Nations report.
On the Israeli side 73 people were killed, of them 67 soldiers.
A July 21 air strike on a five-storey building west of Gaza City demolished Qassas's home, the unemployed 40-year-old recalled.
"Israel killed my pregnant wife and four of my daughters, in addition to five other family members, and it should pay for this at the ICC," he told AFP.
The Palestinians have formed a committee to oversee cases to lodge with the ICC as part of a "national effort" to end Israeli "impunity," including a Gaza committee and another for settlements.
Chief negotiator Saeb Erakat will oversee the mechanism, which includes various figures from the Palestinian political scene, universities and human rights organisations, an official said.
- Demand for justice -
This effort relies heavily on organisations that defend Palestinian rights and collect incriminating evidence.
Qassas approached several of them.
Ahed Bakr, for his part, went to Al Mezan Centre for Human Rights in Gaza to file a complaint against Israel after a strike that killed his son, grandson and nephews as they played on the beach in Gaza City, as witnessed by journalists on July 16.
"I demand the president (Mahmud Abbas) files our case against the terrorist Israel with the ICC to get justice for our children," the 55-year-old fisherman told AFP.
"Israel killed our children on purpose, it wasn’t just one rocket, but four," he said. "The whole world saw this live."
"I will never rest until I see Israeli leaders behind bars," he added.
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The Israeli army is holding its own investigations, with approximately 80 currently open.
Among the cases are the shelling of a UN school on July 24 that medics said killed at least 15 people, and the July 16 bombing of a beach where the four children died.
The arm probes standard "should meet that of any others," said Major General Dan Efrony, the Israeli military's top adviser.
In the face of Palestinian legal charges, Israeli Prime Minister Benjamin Netanyahu has called Israel's army "the most moral in the world".
- Israel vs Hamas-
To Israel's mind, Hamas -- the Islamist movement which de facto rules Gaza -- is guilty of war crimes for launching rockets at Israeli civilians and using Palestinians as human shields.
In a recent report, Amnesty International accused Palestinian armed groups of war crimes for indiscriminate rocket and mortar attacks that killed civilians in both Israel and Gaza.
In earlier reports the rights group also accused Israel of war crimes.
Palestinians do not consider Israeli investigations credible.
"We have documented hundreds of cases in which Israelis are liable to prosecution for war crimes," Issam Younis, director of Al Mezan in Gaza, told AFP.
Younis, a member of the ICC committee, said the aim of joining up was to bring accountability to "a state that thinks it's above the law".
Among the cases brought before Al Mezan were the July 12 bombing that hit a centre for the handicapped in the northern town of Beit Lahiya.
Ola Washahi, 30, and Suha Abu Saada, 47, who had severe physical and mental disabilities, were killed in the strike, which demolished the facility.
Its director Jamila Alaywa turned to Al Mezan.
"We demand justice, even if it takes years at the court," she said. "We are not in a hurry."