MENDON, Ill. (AP) _ A woman drowned her oldest daughter in a bathtub, shot her three other children to death, then killed herself, authorities said.

''I've been in law enforcement for 19 years and this is the most gruesome thing I have ever seen,'' Sheriff Robert Nall said after Wednesday's grim events in this tiny town near the Missouri border.

The bodies were discovered when the father, Greg Epker, returned home from work at a sign company in nearby Quincy, authorities said.

Epker, 32, found that his oldest daughter, 8-year-old Casey, had been drowned; that daughters Shannon, 5, and Mandy, 2, and his son, Lance, 1, had been shot, and that his wife, Sheila, 27, had shot herself, Nall said.

Epker was being treated for shock at St. Mary's Hospital in Quincy, Nall said.

The killings apparently occurred Wednesday afternon, Nall said. All but the eldest child apparently died without a struggle, he said.

''At this point, we have found no motive,'' Nall said. ''It was a spontaneous event, and we don't know what happened. We may never know, exactly.'' The sheriff said Epker is ''completely mystified'' as well.

Mrs. Epker apparently first tried to strangle her oldest daughter before drowning the child, Nall said.

Authorities believe the woman then took her other children into the kitchen, where she laid out pillows and blankets and turned on a gas stove in an attempt to asphyxiate the family.

''We believe that because that didn't work quickly enough, because of despair, she took them into the living room, one by one, and shot them'' with a handgun their father kept on a closet shelf out of their reach, Nall said.

Mendon, with a population of nearly 1,000, is nearly 140 miles northwest of St. Louis.

''We've got a grocery, a gas station, a bank and variety store - that's pretty much Mendon, except for one oil company that services the local area,'' said Mayor Gene Rogers.

''It's just a small town, where everything's based on agriculture and everybody pretty much knows everybody else.''

''You just can't imagine anyone doing anything like that,'' said Ron Durk, who owns the town's bowling alley and lives a few yards from the Epkers. ''Something had to snap.

''We didn't know them too much,'' Durk said. ''We'd just see her out there in the yard with the kids in the summertime. ... Something like this is unbelievable.''

Neighbors said the family had lived in Mendon for about seven years.

''They were sweet little children,'' said Durk's wife, Doris. ''I used to see the little girls out in the yard, and they would wave at me.''