Federal safety regulators are expanding their investigation of Chrysler's Jeep Grand Cherokee and two other Jeep vehicles they started nearly two years ago. At issue is whether millions of Chrysler-made sport-utility vehicles face an unduly high risk of catching fire in rear-end collisions.The National Highway Traffic Safety investigation, launched in August 2010, initially focused on 3 million Jeep Grand Cherokees produced between 1993 and 2004. The agency has now added another 2.1 million 1993 to 2001 Jeep Cherokee and Jeep Liberty SUVs.