His father was among those killed at the al Rawdah Sufi Mosque in Bir al-Abed on Friday.
"No one got out of the mosque," said the survivor, who did not want to be named, speaking outside the Ismailia General Hospital, where other family members and friends were recovering from injuries. "The military could hear the gunshots from their unit and they didn't move. Even the ambulances came under fire."
CNN can not independently verify the military locations in the area, which is closed to foreign and most Egyptian journalists. At the hint of criticism, hospital volunteers ushered in an army officer to move out journalists.
Some of the gunmen wore masks and at least one was carrying an ISIS flag, the state prosecutor said in a statement. At least 27 children were killed and at least 128 people were injured, the prosecutor added.
Another survivor, who did not want to be named, said he hid under dead bodies to avoid detection by the gunmen, who were searching the mosque for survivors.
Wearing a white patch over his eye after being struck by shrapnel, he sat on a sidewalk outside of the emergency room at Suez Canal University Hospital, flanked by his tribesmen.
He lost his brother and nephew during the attack, he said. His son, 14, suffered a broken leg.
"I only found out about my son hours later," he said.
Outside the two main hospitals in Ismailia, which is northeast of Cairo and about a two-hour drive from the northern coastal city of Bir Al-Abed, Bedouin men sat on colorful blankets awaiting news of their injured relatives. They do not want to be identified because they're scared of ISIS, who they fear could target their families in the troubled peninsula.
"They could kill us," one survivor said.
In Ismailia, the local community rallied to help the victims of the attack. Nine months ago, they also helped Coptic families driven by ISIS from Al-Arish in Sinai.
Wessam Hassouna, a local volunteer, said that people had flocked to donate blood, with more than 15,000 bags already collected.
The al Rawdah Sufi mosque is known as the birthplace of an important Sufi cleric. Sufism is a mystical branch of Islam that some jihadists consider heretical.
It wasn't immediately clear if that was the reason the mosque was targeted.
The nation has set aside three days of mourning and President Abdel Fattah el-Sisi has ordered the armed forces to build a memorial to those killed at the mosque, a statement on his Facebook page said.
Ahmed El-Tayyeb, the grand imam of Al-Azhar and one of the foremost clerics in Egypt, condemned the "barbaric attack" and said Egyptians would prevail over terrorism with solidarity and determination.
Al-Azhar, home to a 1,000-year-old mosque and university, is considered the premier religious authority in Egypt, and El-Tayyeb is a prestigious figure in Sunni Islam. Egypt's Muslim are predominantly Sunni.
Gamal Awad, a senior preacher at the Ministry of Religious Endowments, said the only way to fight terrorists is to kill them.
Despite efforts by the ministry to raise awareness through mosques, there is no way to debate terrorists' convictions, he said.
"They will kill again."