Updated

WASHINGTON - U.S. safety officials are investigating whether gas tanks on Jeep Grand Cherokees can cause fires in rear end crashes or rollovers.

The preliminary investigation, begun Monday by the National Highway Safety Traffic Administration, is the first step in determining whether a recall of the popular Chrysler SUV is necessary. The investigation covers three million Grand Cherokees from model years 1993 to 2004.

Advocacy group Center for Auto Safety has asked NHTSA to review whether the gas tank's position below the rear bumper and behind the rear axle could cause fuel to spill if the SUV were struck from behind. In rollovers, a lack of proper shielding for the plastic tank could cause it to puncture, the group said. The neck of the fuel tank could also tear off.

"This is a terrible design," said Clarence Ditlow, head of the Center for Auto Safety. Ditlow said he planned to ask Chrysler to issue a voluntary recall of the Grand Cherokee.

While the agency has not reached any conclusions, an initial review of crash data submitted by auto manufacturers showed that the Grand Cherokee did not have significantly more fires after crashes than other vehicles, NHTSA said.

Chrysler spokesman Michael Palese said the company is cooperating with the government investigation and that the Grand Cherokee has an excellent safety record. The automaker moved the tank's position after the 2004 model.

Chrysler has sold just under 3.6 million Grand Cherokees since the midsize SUV was introduced in 1992, according to Ward's AutoInfoBank. The company started selling a redesigned 2011 model recently.

NHTSA has found 44 Grand Cherokee crashes and 55 deaths since 1992 where fire was listed as the most harmful factor. Of those figures, 10 crashes and 13 deaths were most likely associated with rear end crashes, the federal safety agency reported.