SPRINGFIELD, Ill. (AP) _ A 43-hour hostage siege ended in tragedy when a ''mentally deranged'' gunman killed his girlfriend, critically wounded her sister and then fatally shot himself, authorities said.

The bodies were discovered Sunday evening after police fired tear gas into the heavily barricaded house.

''We thought that it got to the point after 40 hours of negotiating that we had done all that we could and we were concerned about the lives of the two remaining hostages,'' said Police Chief Mike Walton.

Police fired the first of two tear gas volleys into the house about 20 minutes after the gunman, Douglas ''Dougie'' Thomas, had discarded the phone he had been using to talk with police, Walton said.

''The negotiations got to the point where it wasn't negotiating,'' Walton said. ''He never had any demands. He never had anything. He was mentally deranged.''

Police rushed the house after they heard Thomas singing, Walton said. Two wounded hostages were found by the front door under several quilts, the chief said.

Thomas and his girlfriend, Karen Lambert, both 20, were dead of gunshot wounds when they arrived at St. John's Hospital, said Sangamon County Coroner Norman Richter.

Ms. Lambert's 14-year-old sister, Nicole, was in critical condition today after surgery for a gunshot wound to the head, hospital spokeswoman Agnes Hayner said.

During the standoff, Thomas released unharmed his girlfriend's 4-year-old daughter, Katrina Lambert, after police cut off electricity and gas to the house, said police Cmdr. Dave Searcy.

Another hostage, Kelisa Lambert, the 2 1/2 -month-old daughter of Thomas and his girlfriend, was released Saturday, police said.

Police did not know what led Thomas to take the hostages, Walton said.

Heavily armed police had surrounded the house, which Searcy said is the home of the Lambert sisters and their mother. More than 30 families were evacuated Saturday from neighboring houses.

Police were called to the house late Friday in response to a report of a domestic disturbance.

Police spokesman Neil Williamson said Thomas was hospitalized recently for depression, and police believe he had been taking medication that might have affected his moods.

Thomas spent a year in prison for shooting two men at a dance in 1985. A judge reduced charges of attempted murder to two counts of aggravated battery, because a question existed about whether Thomas intended to kill the men.