Fragments of 10 to 20 human skulls have been discovered in a burned-out house in a small town in New Mexico, raising unsettling questions about who the bones belonged to and how the people died, investigators said this week. The Lea County sheriff, Corey Helton, said that the bones had been discovered this month inside a dilapidated property in Jal, N.M., which is home to about 2,200 people in the arid southeastern corner of the state, by the Texas border.