One woman was seriously injured in the attack that rattled a northwest neighborhood.
"Who would do something like this? What kind of animal is he?" Maria Gomez, a neighbor who did not know the victims, told the Denver Post.
The men came to the door of the apartment in Denver's Sunnyside neighborhood on Thursday night, police spokesman Sonny Jackson said. A woman who answered the door was sprayed with a liquid by one of the men and another threw a lighter on her, setting her afire, said Raquel Lopez, another police spokeswoman. The men then set fire to two men and another woman inside the home.
Duane Ramirez, 50, said he was in the backyard of his niece's house, where the attack occurred, when he heard someone yelling repeatedly, "They got a torch."
Moments later, the victim ran out the back door screaming and rolled around in the grass in a desperate attempt to put out the flames, he said.
"They doused her in her face and the front of her body," Ramirez said, adding that he smelled petroleum after the attack.
The flames were out when firefighters arrived. The woman who was doused at the door was in serious condition at University of Colorado Hospital. The other three victims suffered minor injuries. Two young children inside the home escaped through a back door, said neighbor Mary Abdullah said.
"Somebody got the kids out," Abdullah said, adding that she lent her neighbors some coats because their coats had gasoline all over them.
Lopez confirmed that two children were in the house, but she didn't know their ages. A motive for the attack has not been released
The attack occurred in a sprawling complex of brick row apartment buildings next to warehouses and older detached homes.
Police were also investigating a check cashing store robbery earlier Thursday in which two men used a flammable liquid to set a small fire at a customer service window. In that instance, the suspects used a similar pump sprayer, Lopez said.
No one was hurt, and it wasn't known if the incidents were related. The store is about three miles east of the apartment complex.
An employee at the check cashing business said Friday she was not allowed to comment about the robbery. Burn marks were still visible on the glass that separates employees from customers, and a wall behind the clerk's desk was blackened by flames.