The investigation into a fiery crash on Interstate 66 in Fairfax County on Tuesday that killed a 2-year-old boy, critically injured his pregnant mother and killed her fetus focused yesterday on why the gas tank in the family's Geo Metro exploded when rear-ended, a statistically rare occurrence.

The four-vehicle chain-reaction crash killed George Zak Marchman, 2, of Fairfax County. His mother, Elizabeth Ann Marchman, 33, remained in critical condition yesterday in the intensive care unit of Washington Hospital Center.

Marchman was driving the Geo Metro when it was rear-ended by a Chevy pickup driven by Matthew R. Cable, 21, about 9:45 a.m. on I-66 just west of Route 123, police said. The Geo then struck a Buick, which hit a Toyota, closing the westbound lanes of I-66 for two hours and snarling secondary roads. Five other people were injured, including two retired Fairfax firefighters who tried to rescue the Marchman family from the burning vehicle.

Cable, of Hagerstown, Md., was cited for reckless driving. Police said he was distracted while trying to read a map.

When Cable's pickup truck struck the Geo Metro, the car's gas tank ruptured and the gasoline ignited, fire investigators said. But it will take weeks, and perhaps months, of painstaking investigation to determine why.

"You don't hear of such a thing very often," said Renee Stilwell, a Fairfax County Fire Department spokeswoman. "Cars are made so the tanks don't explode. Obviously, something didn't work right in this one."

A neighbor who answered the phone at the Marchman home said family members declined to comment.

Statistics compiled by the federal government and industry groups show how unusual the explosion was. In 2002, only 102 of the 32,480 vehicle occupants who died in crashes nationwide were killed by a rear-end impact that resulted in a fire, according to the Insurance Institute for Highway Safety, an Arlington-based research group funded by insurance companies.

"It's a very rare occurrence," said Rae Tyson, a spokesman for the National Highway Traffic Safety Administration. Federal law mandates that vehicles sold in the United States be able to withstand a 50-mph rear-end impact without an appreciable loss of fuel.

Tyson said auto safety has increased dramatically in the decades since the federal government recalled the Ford Pinto in 1979 after explosions.

But accidents can still trigger explosions a number of ways, especially in high-speed crashes, Tyson said. For example, if the filler neck -- a metal tube that connects the gas cap to the gas tank -- is struck with great force, it can separate from the fuel tank, causing fuel to spill. That happened in a recent crash in Kansas, where a Ford Crown Victoria sitting on the shoulder was hit by a truck and exploded.

The National Highway Traffic Safety Administration completed an investigation last year into a number of such explosions involving Crown Victorias that were police vehicles. The agency concluded that the car met or exceeded federal standards.

Officials and industry groups said they knew of no investigations of the Geo Metro involving fuel tank problems. The compact car was manufactured and sold by General Motors between the late 1980s and mid-1990s.

James Schell, manager of product safety communications for General Motors, expressed his condolences to the Marchman family yesterday but said the Geo Metro "is a safe vehicle."

As rare as vehicle explosions are, they are much more deadly than other crashes, experts said. "People are stunned when a crash occurs, and you don't realize how fast you have to get out of your belt and get out of the car," said Joan Claybrook, president of the consumer group Public Citizen and former administrator of the National Highway Traffic Safety Administration.

"You're talking seconds before you are engulfed," she said. "And this woman was pregnant and had a child in the car, so it was even harder."

Police said yesterday there are no plans to upgrade the charges against the driver of the pickup.

"This was just a tragic accident, and that's all it was, an accident," said Sgt. Wallace Bouldin, a Virginia State Police spokesman. "There is no evidence to indicate that this young man did anything other than look down and read a map and just made a mistake. We've all done different things that take our attention from the roadways."

Bouldin said Cable was not driving at an "excessive speed," though he could not say how fast he was going or provide other details, such as whether the Geo Metro was stopped when hit.