DENVER: Police say four people were set on fire during an attack at a northwest Denver home.

Police spokeswoman Raquel Lopez said two men knocked on the door of an apartment at about 7:30pm on Thursday and sprayed the woman who answered with a flammable liquid and tossed a match or lighter at her.

The men then entered the home and set two males and another female on fire, KMGH-TV, Denver, reported.

The woman who answered the door suffered the most serious injuries and was being treated at the University of Colorado Hospital, The Denver Post reported.

Three other adults in the home also were sprayed flammable liquid and set afire.

Police the matter is under investigation and a motive had not yet been determined.

The two suspects are being sought.

“This is something I’ve never seen in my years ... someone set someone afire like this,” said Denver police spokesman Sonny Jackson, quoted by ABC News Radio. “This is highly unusual.” Police investigated whether the incident was related to an attempted robbery at a check-cashing service the same night 3 miles away.