Investigators believe holes in the top of a muffler ignited the floorboard and interior of a vehicle on Interstate 86 in the town of Erwin on Tuesday, killing a 4-week-old infant and her 18-month-old sister. The infant, Sinaya Beam, and her sister, 18-month-old Izzybella Beam, both died in a fire that broke out in the back seat area of the 1999 Kia Sportage driven by their mother, 25-year-old Melissa Johns of Savona.
          Johns was driving home from Guthrie Clinic in Sayre, where the infant had her first checkup Tuesday, family members said. State Police Investigator Chris Wilkinson said investigators think the fire started when welded spots on the top of the muffler eroded, allowing hot exhaust gases to escape, burn through the floorboard and ignite the interior of the car. "That coincides with witness accounts that said the fire was behind the driver's seat," Wilkinson said. "It's hard to get any information from the mother because she's so distraught."
          Wilkinson said diaper bags and other items in the back seat caught fire as the flames spread. The source of the blaze was determined after investigators disassembled the muffler at Crane's Towing Inc. on Addison Road in Painted Post, Wilkinson said. The fire started in the back seat area of the vehicle shortly before 5:30 p.m. Tuesday as Johns drove west between Exit 43 (Painted Post) and Exit 42 (Coopers Plains). Police said she pulled off on the south shoulder of the road and stopped in an effort to get the children out.