More than 14 months after the Agriculture Department began an investigation into how the U.S. supply of long-grain rice became tainted with an unapproved genetically engineered variety -- an event that continues to disrupt U.S. exports -- the government announced yesterday that it could not figure out how the contamination happened. Agency officials said documents from several years ago that might have helped them determine what went wrong had been lost or destroyed. Lacking clear evidence of who was responsible, they said, the government will not take enforcement action against any person or entity, including Bayer CropScience, the company whose gene-altered products slipped into the food supply. The widespread, low-level contamination with experimental genes that make the rice pesticide-tolerant, one of several such events in recent years, prompted countries around the world to cut off imports of U.S. long-grain rice. Rice prices plummeted, and many farmers, scientists and biotechnology activists called for an overhaul of the oversight system for gene-altered crops. While some countries have begun to accept U.S. rice with added testing, the European Union and Russia have not -- a trade loss valued in the hundreds of millions of dollars a year. Critics assailed the report as yet more evidence that the nation's regulatory system for gene-altered crops is broken. This underlines the anxiety people have about more such incidents occurring," said Margaret Mellon of the Union of Concerned Scientists, a science-based advocacy group that has called for a more rigorous approval process for biotech crops.