Up to 5.1 million Jeep Grand Cherokee, Jeep Cherokee and Jeep Liberty model SUVs may be subject to one of the largest recalls in history, depending on the results of an engineering analysis by the National Highway Traffic Safety Administration (NHTSA).

The investigation centers around whether the Jeep models in question are subject to a higher rate of bursting into flames in a rear-impact accident than other similar vehicles in the same class. The NHTSA upgraded the investigation on Thursday to include three models of Jeep vehicles for model years 1993 to 2004.

NHTSA's assessment of the data collected ...  indicates that rear-impact-related tank failures and vehicle fires are more prevalent in the [Jeep Grand Cherokee] than in the non-Jeep peer vehicles, an investigation summary from the NHTSA said Thursday.

The investigation is focused on the fuel tank system integrity of the cars. Originally only Jeep Grand Cherokee vehicles were under investigation for the possible flaw, but the NHTSA has now expanded the investigation.

The Center for Auto Safety has been petitioning for a recall of the vehicles and said recently in an open letter to the NHTSA that in NHTSA's history of defect investigations and recalls, there has never been one where 4-year-old children in child restraints have burned to death in fire crashes until now. The letter continues to detail two instances where Jeep Grand Cherokee vehicles were rear-ended and burst into flames, consequently killing two children.

In an earlier letter to Fiat S.p.A. (Milan: F) CEO Sergio Marchionne, the Center for Auto Safety referred to the Jeep Grand Cherokee as a modern day Pinto for soccer moms. As with the Pinto, the fuel tank is located behind the rear axle: a dangerously vulnerable area in the rear impact crush zone. Chrysler LLC which makes Jeep brand vehicles is a subsidiary of Fiat. The Center for Auto Safety goes on to assert that there have been 184 fatal fire crashes in model year 1993 to 2009 Jeep Grand Cherokee vehicles and that 269 people have died as a result.

Since the notorious Ford Pinto's fiery demise in 1970s, standard practice has been to place the gas tank ahead of the rear axle, but this best practice was not followed in the design of the three Jeep models from the era under investigation.

If the 5.1 million Jeep models under investigation are eventually subject to a recall it would be one of the 10 largest recalls in history, according to Bloomberg News. A recall of that size could become a significant expense for Chrysler, a particularly worrisome development for Fiat, which has depended on Chrysler for profitability in recent years.

Fiat S.p.A. shares fell 0.46 percent to €3.47 Thursday.