A widening investigation has implicated some of the world's largest oil services firms in an alleged scheme to bribe Nigerian officials to win $5.3 billion in contracts to build a natural gas complex in the African country. As details of the case unfolded, one of the key players, Houston-based Halliburton Co., said that the questionable conduct occurred almost entirely before it became a partner. But information shows that Halliburton, through a subsidiary, was more involved in some of the suspicious deals than it has acknowledged.