The gas cylinder exploded in a building believed to be an auto parts shop about 6:30 p.m. The blast was in a largely vacant industrial zone about a mile northeast of the University of Southern California campus.
Ayman Eldik, the manager of a neighboring market, said the explosion felt like the whole building was going to come down. He said two women ran out the back door of the auto shop through a shared alleyway and banged on the back door of his market. He said they were distraught and injured.
"I heard girls screaming, 'Please help, help,'" he said. "I look to the girls, really bad. Blood everywhere."
The women and two other men, all believed to be in their early 20s, were transported to local hospitals. Two of them were in grave condition and one died at the hospital, officials said.
Authorities were tight lipped about the exact cause of the explosion or what triggered it, only saying there were cylinders involved.
"We have an idea what the cause was, it's still under investigation and we're working collaboratively with LAPD ," said Mario Rueda, deputy chief of the Los Angeles Fire Department.
Officials did not confirm if criminal activity took place. They described the scene inside as chaotic.
"The scene of the explosion looks ransacked, there's debris and materials all over the ground," Rueda said.
Officials said there was no fire and no hazard to neighboring businesses.