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."

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.