Prosecutors in China have filed charges against British investigator Peter Humphrey and his American wife, the official Xinhua news agency said on Monday, after the couple were detained last year following work they did for British drugmaker GlaxoSmithKline Plc. The case against Humphrey and his wife Yu Yingzeng has become a key piece in a long-running investigation into GSK, whose China executives have been charged with orchestrating a widespread network of bribery to promote sales.