Then he saw two small blackened bodies in the wreckage of his home.
"Those were my children," he sobbed, collapsing in anguish amid the charred corrugated iron sheets and twisted metal.
Mwangi had been feeding his cow when the call went out around 9 a.m. — a section of pipe had burst near the river that cuts through the slum and gasoline was pouring out. Men, women and children grabbed pails, jerry cans, anything they could find to collect the flowing fuel.
Mwangi had planned to get a bucket and join them — he'd done so before with earlier diesel leaks without any problem, he said, and a bucket of fuel could pay a month's rent. "Everybody knows that fuel is gold," the 34-year-old said.
But before he could join the others, an explosion rocked the area, sending a fireball racing through the Sinai slum in Nairobi's industrial zone. Screaming men and women in flames desperately jumped into the river and a nearby sewage ditch, but fuel had leaked into the rancid water and in many places it caught alight.
Red Cross co-ordinator Pamela Indiaka said at least 75 bodies had been recovered and the death toll was expected to rise. At least 112 people were taken to hospitals with severe burns.
"I've lost count of the number of bodies," said police official Wilfred Mbithi as he stood at the scene, where burned corpses floated in the waste water for most of the day. Nearby, a young woman in jeans clawed at a smouldering pile, shrieking with grief.
Hours after the blast, charred skeletons lay on the ground as firefighters battled to control the flames and shocked, weeping residents wandered through the wreckage.
Survivors told of hellish scenes as flames leaped through the slum's twisting, filthy alleyways filled with people. Some burning victims fled into nearby homes igniting bed linen or clothing.
Michael Muriuki said that's what happened to his wife and three children when neighbours, their clothing aflame, ran into their shack while he was at work. His wife and two of the children escaped, but his 5-year-old daughter was trapped. Muriuki found her smouldering body when he returned home.
"Her name was Josephine," he said brokenly.
Monday's blast was not the first time scores of poor Kenyans have died in a terrible blaze while scooping up spilled fuel. In 2009, at least 120 people were killed after a huge crowd descended on an overturned gasoline tanker, which then blew up. But poverty-stricken families say they have little choice: spiraling food and fuel prices mean many cannot even feed their children.
The head of the state-owned Kenya Pipeline Company, which operates the stricken pipeline, said the gasoline leak was caused when a gasket burst due to pressure buildup in the pipe. The cause of the explosion was not immediately known, though garbage fires are common in the area.
"It was a technical problem due to overpressure," said KPC head Selest Kilinda, speaking on national television station NTV. "The pressure must have been very robust to tear a gasket."
At least 112 burn victims arrived at Kenyatta National Hospital, where there was in urgent need of blood donors and blankets, said director Richard Lisiyampe. Many children were among the victims, and most had burns covering more than a third of their bodies, he said.
Inside the hospital, beds were crowded together and doctors and nurses rushed from victim to victim, many of whom had strips of skin peeling from their heads and bodies. One man picked at his hands distractedly, peeling off skin like gloves.
"Every place was full of fire even in the water. My survival is a miracle," said 27-year-old George Njoroge, who escaped by jumping in the sewage and was lying in bed with burned hands and a bandaged head.
"I asked my wife about (our children) but she doesn't know their fate."
Prime Minister Raila Odinga visited the hospital and said the government would cover medical expenses for the injured and compensation to those who lost loved ones.
"It is terrible, terrible, terrible," said Odinga. "There will be a proper investigation."
The Red Cross set up two tents for first aid and counselling and was providing body bags and materials for temporary shelter.
If the explosion had happened in a rich part of the capital, said resident Evans Makali, the government would be sued. But since it happened to poor people, nothing would change, he said. Activists say Kenya's government is notoriously corrupt and donors have condemned officials for manipulating food prices and skimming millions of dollars from programs meant to help the poor.
Associated Press writers Katharine Houreld and Malkhadir M. Muhumed contributed to this report.