The U.S. National Highway Traffic Safety Administration (NHTSA) has intensified an investigation into over two million General Motors cars after 600 complaints of engine problems that could lead to a fire. According to a report from the Reuters news agency, the NHTSA GM investigation covered about 2.3 million Buick, Chevrolet, Pontiac and Oldsmobile models built between 1996 and 2000 with 3.8 liter V6 engines. According to Reuters, NHTSA said 604 consumers had complained either to GM or the agency that an engine backfire had fractured the intake manifold, with 44 reports to NHTSA of fires caused by fuel leaking from the broken manifolds. Reuters said GM had recalled 276,000 V6-powered cars in 1996 for a similar problem, and NHTSA said the car maker had made three design changes to models built since then aimed at dealing with backfiring. NHTSA also said the number of complaints had been rising over the past six years, Reuters added. According to Reuters, the NHTSA said GM was contending the problem was not a safety concern because it was rare, there were few reports of injuries and the problem happens during start-up, not while the vehicle is in motion. Reuters said NHTSA’s move made the investigation an engineering analysis, one step away from a recall and added that NHTSA rarely orders car makers to recall vehicles; either car makers issue their own recalls, or the investigations find no safety defect.