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The National Highway Traffic Safety Administration has upgraded an investigation into certain Jeep models to an engineering analysis, according to Bloomberg Businessweek. NHTSA began investigating certain Jeep Grand Cherokee, Cherokee and Liberty models with fuel tanks positioned behind the rear axle in late 2010. Regulators say fires tied to rear-impact crashes may be responsible for 15 deaths and 46 injuries. The results of the engineering analysis will determine whether or not Chrysler is required to issue a recall for the defect. If that happens, the automaker could be looking at recalling 5.1 million vehicles, making the campaign one of the largest in U.S. automotive history.

Chrysler says it's uncertain how many of those models are still on the road today, though it is certain NHTSA will find the SUVs neither defective nor unsafe. Even so, that hasn't stopped some consumer advocacy groups from lambasting the vehicles. The Center for Auto Safety proclaims that the vehicles are a "modern day Pinto for soccer moms." Chrysler has analyzed crash data in Grand Cherokee models and says it has found no greater incident of post-accident fire than in vehicles produced by other manufacturers.

The investigation covers 1993-2004 Grand Cherokee, 1993-2001 Cherokee and 2002-2007 Liberty models.