Four people were sprayed with flammable liquid and set on fire inside an apartment building, Denver police told ABC News Radio.
Two men knocked on the door to an apartment in North Denver, and then attacked its occupants with a garden sprayer and flame, police told ABC News affiliate KMGH.
"Two men came to the door of the residence and said some words to the people inside then sprayed an accelerant on them and then ... set them afire," Denver police spokesman Sonny Jackson told ABC News Radio.
"Three of the individuals have minor, non-life threatening injuries," he said. "One has more serious injuries and is being taken to an area hospital to be treated."
Police still were investigating the attacks, and did not yet have a motive or a detailed description of the suspects, Jackson told ABC News.
"This is something I've never seen in my years ... someone set someone afire like this," Jackson said. "This is highly unusual."