A 19-month-old infant injured in an explosion at a Pomona shopping center has died, coroner's officials said. Lyndsay Martinek died at the Sherman Oaks Community Hospital Burn Unit on Sunday, said coroner's spokesman Bill Gold. Gold said no official cause of death was released pending an autopsy. The girl was injured July 10 when a transformer exploded at the Indian Hill Mall. Also injured in the explosion were the girl's mother, Joy Lynn Williams, 25, her step-brother, Aaron Williams, 8, and step-sister, Shawna Williams, 7. Both Joy Lynn and Aaron Williams remained in critical condition at the burn center, but Shawna Williams was expected to be released today, hospital spokeswoman Johna Rogovin said.