The National Highway Traffic Safety Administration has expanded an investigation into the Jeep Grand Cherokee to cover a total of 5.1 million vehicles. After reports of fire risks in the 1993-2004 Grand Cherokee, NHTSA is now conducting an "engineering analysis" that also covers the 1993-2001 Jeep Cherokee and 2002-2007 Jeep Liberty.

NHTSA first stated looking into reports of fires in the 1993-2004 Jeep Grand Cherokee about two years ago. The SUVs came under more scrutiny last summer, when the Center for Auto Safety reported that 2.2 million vehicles could have a defective fuel tank. The plastic fuel tank is located between the rear axle and the rear bumper; in a rear-end collision, it could rupture and start a fire. Bloomberg reports that a representative from the Center for Auto Safety called the cars, "a modern day Pinto for soccer moms" -- referring to the 1970s Ford compact car also known for fire risks.

The investigation has now expanded to also cover the 1993-2001 Jeep Cherokee and 2002-2007 Jeep Liberty, a total of 5.1 million vehicles. Jeep parent company Chrysler says it will fully cooperate with the investigation, but asserts that the models "are at no greater risk of exposure to fire in rear end collisions than peer vehicles."

The NHTSA says there have been 26 fires, 46 injuries, and 15 fatalities reported from fires caused by the affected vehicles. Chrysler reportedly doesn't yet know how many of the 5.1 million vehicles are actually still on the road -- some of them where built nearly 20 years ago.No vehicles have been recalled so far, but if the NHTSA investigation reveals a serious safety defect, it could lead to a recall of the Jeeps.

Earlier this year, Jeep recalled some versions of the 2010 Wrangler to address a problem with a build-up of debris that could cause a fire.

Sources: Chrysler, NHTSA, Bloomberg