ROCKVILLE, Ind., May 21 (Special) —A Parke County grand jury today began investigating a fire which killed six mentally retarded children at a home here May 3. The grand jury will seek to determine if negli gence contributed to the tragedy. The State Fire Mar shal’s office reported the negligence was a principal factor in the rapid spread of flames which gutted the structure killing six of the 11 children of the state approved home. Mr. and Mrs. William Johnson, operators of the home, were among a number of persons subpoenaed to testify as the grand jury opened its investigation. The grand jury was called to investigate the fire following a report by the State Fire Marshal’s office that two teen-aged babysitters failed to summon fire men immediately, permitting the flames to engulf the flame home. The home for retarded young sters was licensed by the State Welfare Department. Bernard M. Sweeney, chief arson inspector, said although the fire started from accidental causes, two large kerosene cans next to a faulty dryer, and the failure of quick action by the teenage boys, permitted the fire to engulf the building before firemen arrived. _ Among those subpoenaed by Parke County Prosecutor Earl e Dowd to testify were officials of Continued On Page 5, Col. 5.