Mounir al-Jarah slowly takes down the bricks he used to wall up the entrance to his sister's courtyard. Inside, flesh still clings to the walls; blood-soaked furniture and family items lie broken and mangled.

Mounir's eyes search around the old house as he recounts the events of 16 January, when a rocket fired from an unmanned aerial vehicle killed his sister, her husband and four of her children.

Sitting around drinking tea with the family in their small courtyard, Mounir heard the loud buzzing of an Israeli drone, clearly visible in the sky above.

He went inside for a moment and, as he returned, he saw a ball of light hurtling down toward him. There was a loud explosion and he was thrown backward. He gathered himself and stumbled out into the courtyard, where he saw the scene he says will never leave him.

"We found Mohammed lying there, cut in half. Ahmed was in three pieces; Wahid was totally burnt – his eyes were gone. Wahid's father was dead. Nour had been decapitated. We couldn't see her head anywhere."

All six members of the family had been blown to pieces, coating each wall of the narrow enclosure with blood and body matter.

"You cannot imagine the scene: a family all sitting around together and then, in a matter of seconds, they were cut to pieces. Even the next day we found limbs and body parts on the roof, feet and hands," Mounir says.

Fatheya, 17, is one of the few surviving members of the family. Slipping further into grief-stricken madness, flitting from one horrific description to the next, she says: "There were rocks and dust and fire … It's very difficult … I can't, no matter how I try to explain my situation to you, picking up the pieces of my dead family … I couldn't handle it, limbs and flesh all around me. What have we done to deserve this?"

The attack on this home in Gaza City is just one of more than a dozen incidents recorded by Amnesty International where Israel's unmanned aerial vehicles (UAVs) – or drones – killed one or more civilians.

During the 23-day offensive, 1,380 Palestinians perished, 431 of them children, according to figures published by the World Health Organisation.

A Guardian investigation into the high number of civilian deaths has found Israel used a variety of weapons in illegal ways. Indiscriminate munitions, including shells packed with white phosphorus, were fired into densely populated areas, while precision missiles and tanks shells were fired into civilian homes.

But it is the use of drones in the killing of at least 48 civilians that appears most reprehensible.

The drones are operated from a remote position, usually outside the combat zone. They use optics that are able to see the details of a man's clothing and are fitted with pinpoint accurate missiles.

Yet they killed Mounir's family sitting in their courtyard, a group of girls and women in an empty street, two small children in a field, and many others.

Chris Cobb-Smith, a senior military analyst in Gaza to advise Amnesty in its investigation, is at a loss to explain how these killings occurred.

"With a weapons system that is so accurate, and with such good optics, why are we experiencing so many civilians being killed? There should be no excuse for these numbers," he said.

The Guardian asked the Israeli Defence Forces (IDF) about their use of armed drones but they declined to be interviewed on the subject. Instead they issued a written statement: "The IDF operated in accordance with the rules of war and did the utmost to minimise harm to civilians uninvolved in combat. The IDF's use of weapons conforms to international law."

Israel still refuses to confirm whether it is using armed drones over Gaza but, in the online version of an Israeli army magazine, Major Gil, the deputy commander of the first UAV squadron, describes using the drones to carry out attacks during this offensive.

"We were able to monitor each of the soldiers at any minute and identify any threats to them," he said.

He also describes being able to clearly distinguish fighters from women and children and other civilians: "When there were innocent people around, we would wait for the terrorist to leave the child and then hit him," he said.

Lieutenant Tal, an operator and intelligence officer in the UAV squadron, describes the details the drone cameras can see.

"We identified a terrorist that looked like an Israeli soldier. Our camera enabled us to see him very clearly. He was wearing a green parka jacket and was walking around with a huge radio that looked exactly like an army radio. It was very clear he wasn't a soldier."

According to Robert Hewson of Jane's Defence Weekly, who has been monitoring armed drones and their role in assassinations in the occupied territories since 2004, most of Israel's armed drones use a modified anti-tank weapon called a mikholit ("paintbrush" in Hebrew) that delivers a small but intense explosion like the one in Mounir's courtyard.

Teams of human rights investigators and international law experts are now building the case for war crimes charges against Israel and trying to decipher why so many innocents were killed in this offensive.

The Israeli newspaper Ha'aretz discovered that the IDF's international law division (ILD,) the body responsible for advising Israeli forces on the legality of their actions, had authorised an easing of the rules of engagement in Gaza.

A copy of the rules of engagement for Operation Cast Lead was obtained by Ha'aretz in the days before the offensive began. According to a journalist who saw the document, the new, less stringent rules were approved at the highest levels of the Israeli military.

Ha'aretz was repeatedly blocked from publishing the document by the military censor.

In recent days, striking testimony has emerged from Israeli soldiers involved in the Gaza fighting, in which they described the shooting of civilians and the low regard held among the troops for Palestinians.

The soldiers, all graduates of the same Israeli college, gave their testimony in a session in mid-February. It was published last week in the college newsletter, forcing the Israeli military to announce another investigation into the conduct of the war.

In one of the worst cases, a soldier described how an Israeli sniper shot dead a Palestinian mother and her two children. He said the soldiers around him believed the lives of Palestinians were "very, very less important than the lives of our soldiers".

Another soldier said a company commander had given orders to shoot an elderly Palestinian woman who was walking on a road about 100m from a house the soldiers had taken over. Others talked about the influence of military rabbis and the sense among soldiers that they were fighting a "religious war".

Some efforts to explain the large number of civilian deaths have focused on the aggressive conduct of the war by Israel. But other explanations look at a broader shift in military philosophy.

The former head of the ILD, Colonel Daniel Reisner, spoke frankly to the Israeli media in the aftermath about the role the body plays in pushing the boundaries of what is acceptable in war.

"What we are seeing now is a revision of international law," Reisner said. "If you do something for long enough, the world will accept it. The whole of international law is now based on the notion that an act that is forbidden today becomes permissible if executed by enough countries.

"International law progresses through violations. We invented the targeted assassination thesis and we had to push it."

The United Nations human rights council is preparing to launch an inquiry into allegations that Israel committed war crimes during its offensive against Gaza. Israel has already dismissed the inquiry, accusing the UN of bias. It says Palestinian fighters committed war crimes by firing mostly crude homemade rockets into civilian areas.

Amnesty International says Palestinian militants should be prosecuted for war crimes, but insists the vast majority of offences were committed by Israel. "Only an investigation mandated by the UN security council can ensure Israel's cooperation and it's the only body that can secure some kind of prosecution," said Amnesty's Donatella Rovera, who spent two weeks in Gaza investigating war crimes allegations.

"Without a proper investigation there is no deterrent. The message remains the same: 'Its okay to do these things, there wont be any real consequences'."