BISMARCK, N.D. (AP) _ A couple and two of their three children were found shot to death in a mobile home on their farm, and authorities were searching for the missing 15- year-old son.

The bodies of Ronald Neugebauer, 44, his wife, Maureen, 40, their 16-year- old daughter, Michelle, and 12-year-old son, Ryan, were found by a relative Monday at the farm north of Menoken and 10 miles east of Bismarck, said Burleigh County State's Attorney Patricia Burke.

The whereabouts of their other son, Michael, were unknown, she said.

A nationwide hunt was launched for a 15-year-old suspect ''because we believe he was involved,'' Burke said, but did not identify the son as the suspect. The suspect was believed to be driving a white Pontiac.

''We at this time are not sure where he is, or who he has or doesn't have with him,'' Burke said. ''Let's put it this way, if you have possibly just killed four people, what is one more?''

The murder weapon was not found, Burke said. The killings apparently occurred late Sunday or early Monday, she said.

Neugebauer's mother, who lives in a separate house on the farm, was not hurt, Burke said.

Neugebauer raised cattle and grain on a farm that had been in his family for years, neighbors said, and his wife was a secretary at the University of Mary in Bismarck.

''They were real nice people. They just kind of kept to themselves,'' said Keith Salter, who lives with his wife and son a mile down the road.