Bitterly cold, famished, and thirsty they may have been. But the dozens of men, women, children and elderly of the extended Samouni family, sheltering in the unfinished warehouse-like building owned by one of their relatives, at least felt relatively secure.

Some explain that Israeli troops had explicitly told them to remain in the building – in several cases after actually escorting them there. They were relieved to wake to find the neighbourhood quiet for the first time since the ground offensive had began 36 hours earlier. So much so, according to one of them, that some time after 6am, four of the men decided to set out and bring an uncle and his family to their refuge.

It was then, survivors say, that the greatest of all the horrors visited on the Zeitoun district of eastern Gaza City occurred. A shell hit the little group, killing 27-year-old Muhammad instantly, and injuring the other three still lingering at the door.

According to 19-year-old Maysaa, a second shell or missile fell on the building's roof with such force that she would in hindsight assume it came from an Israeli F-16. At the time she lay down with her nine-month-old daughter Jumana under her, listening to the screams of those around her, as the structure filled with smoke and dust. When the smoke began to clear, she looked around and saw what she says were between 20 and 30 bodies, and 20 wounded. The dead included her own husband Tawfiq, her father-in-law Rashed, who "was hit in the head and whose brain was on the floor", and a five-month-old baby "whose whole brain was outside his body".

With foreign journalists barred by Israel from entering Gaza, it is not easy to reconstruct every detail of what happened with any certainty. But Maysaa's account to the Israeli human rights organisation B'Tselem is broadly and quite separately corroborated by other family members, including a religious studies teacher, Ahmad al-Samouni.

The 23-year-old told The Independent: "One shell hit the door, killing my cousin Muhammad immediately. One shell, I believe it was from an Apache, hit the ceiling. Then another shell and another." Ahmad could only recognise his dead mother Rahmeh, 45, from her clothes and earrings because "part of her head was gone".

Ahmad says they counted 25 bodies – 10 adults and the rest children. One of the infants was the five-year-old daughter of his cousin Salah. "Her last words were 'Baba, Baba' and she died," he said. "She was hit in the head."

The teacher rallied survivors, urging them to leave but not everyone could make it. "We were about 50, some of us wounded," he said. "My grandmother Shifa was too sick, we left her." As Ahmad walked to the main road, where they hoped to find cars to take them to the hospital, he tore a piece off his white T-shirt and attached it to a stick as a white flag. Some female members of the Samouni family describe being shot at as they escaped while holding white flags.

Maysaa says she escaped with her daughter and brother-in-law, Musa, to the house of an uncle. There she found at least 40 Israeli soldiers and about 30 Palestinians, including a handful that were blindfolded. The soldiers administered first aid to Maysaa and her daughter before releasing them, but, she testifies, said they would keep Musa and his uncle "in case Hamas came". She adds: "I understood that they intended to use them as human shields."

The Israeli military denies targeting any buildings in the Zeitoun area on Monday and says it never forcibly gathers civilians in a specific building. It is continuing to investigate the incident.

Interviewed at Al Quds hospital yesterday, Ibteffam Al-Samouni said soldiers had "fired missiles" in the neighbourhood on Sunday and broke into five houses inhabited by Samouni family members and told the residents to leave. They had then walked to the warehouse belonging to their relatives. She added: "The soldiers told us: 'Stay there and shut the door, habbibi [my love]."

"We are civilians, we are farmers, we don't take part in any kind of resistance," said the 30-year-old woman.

After the shelling on Monday, she tried to escape. But, she says, there was shooting from Israeli forces as they walked from the building despite the fact that members of the group were waving white flags and shouting in Hebrew: "Kattan, Kattan [We have kids]." One of the men, Iyad, was shot in the leg by soldiers. "They ordered us to keep moving and not take the wounded man," she recalls. It was not until three days later that his body was recovered and brought to the hospital.

Ahmad is strongly critical of Hamas for keeping its leaders in hiding while civilians suffer from Israel's "brutal arsenal of weapons". Wael al-Samouni, the owner of the warehouse that was to become a tomb for his family members, insisted that Palestinians would remain steadfast but added: "I appeal to the world for help. This is a tragedy."

Zeitoun: Site of a slaughter

*Survivors of an Israeli attack in Zeitoun say that up to 30 people were killed in a building where dozens of men, women and children sheltered from the Gaza offensive on 5 January. The Israeli army denies that the families were told by the Israelis to head for the building on the day before the strikes, and that shells targeted the area the next day.