Abdulhameed Alyousef originally thought his nine-month-old twins Aya and Ahmed were unaffected by the air strike and left them with their mother Dalal while he went to help relatives nearby.
But later he found out that all three had died in the incident.
"I left Ahmed and Aya in good health," he said through tears in an interview.
"Why did this happen? I thought they were OK. Now they are gone."
Khan Sheikhoun was hit by three explosions on Tuesday morning, followed by a rocket that hit a hospital treating the injured and hundreds suffered symptoms consistent with reaction to a nerve agent.
Mr Alyousef's house was hit in the second blast.
He said that he heard the first explosion and then left his wife and children to go to see if he could help other survivors in his neighbourhood.
"When I went to the street, I saw the gas was taking effect on them," he said. "I helped them, trying to lift them up -  I wanted to save them. I couldn't go back to my family because I was helping people."
When he reached his parents' house, Mr Alyousef said he found the bodies of his brother Yasser and his children.
"He had been trying to help his son - they died on top of each other. I saw the foam coming out of their mouths, and they were blue."
In total, 22 members of Mr Alyousef's family died in the attack. As well as his wife and twins, he lost two brothers, his parents and several nieces and nephews.
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As he was trying to save one of his nieces, Mr Alyousef began to suffer the symptoms of nerve gas himself and collapsed.
He was taken to hospital where he asked about his own family.
"I asked them 'where are my babies and my wife?'," he said.
Mr Alyousef said that at first he was told his family was alive but later he was taken to see their bodies.
"I left them in the house and they were OK and they brought them to me dead. They were suffocated.
"Why did they kill them? Why with gas? What did they do?"
By Georgina Rannard, UGC & Social news
A grieving father has recalled the last time he saw his nine-month-old twins before they were killed in the suspected chemical attack in Syria.