May 24 (Reuters) - U.S. safety regulators have expanded their investigation into engine fires in the Chevrolet Cruze small cars to a second model year. Documents posted this week on the National Highway Traffic Safety Administration website show the fire probe, which initially covered 2011-model Cruzes, now includes the 2012 model year. The expanded preliminary investigation now covers about 370,000 cars, up from the initial 177,000, GM spokesman Alan Adler said. A preliminary investigation is the first step in a process that could lead to a recall if regulators determine that a manufacturer needs to address a safety issue. There have been no reports of accidents or injuries related to the probe and GM is cooperating with NHTSA, Adler said. GM is investigating a number of fires, but Adler declined to say how many. In a letter dated April 4, NHTSA requested information from GM by May 11. Adler said the company had responded to the agency's questions, but the response had not been posted online by NHTSA. In April, NHTSA said it had opened a probe after receiving two complaints of engine fires in 2011 Cruzes while the cars were on the road. Both cars were destroyed, according to the complaints.