The criminal investigation that opened yesterday into General Motors Co.'s handling of an ignition flaw linked to 12 deaths adds to legal challenges that threaten to overshadow the company's turnaround. GM, the largest U.S. automaker, is now at the beginning of a potentially years-long road dotted with inquiries, lawsuits, government fines and public skepticism. Besides the U.S. Justice Department probe, GM must answer to Congress, the Federal Bureau of Investigation, the Transportation Department and lawyers the company hired to investigate itself.