A Christmas tree that was set on fire caused a blaze in a Philadelphia row house that killed a dozen people, including eight children, last week, the city’s top fire official said Tuesday, citing preliminary findings of an investigation. Philadelphia Fire Commissioner Adam Thiel, detailing early results from the probe, said seven smoke alarms were found in the building but only one of them sounded as the fire spread. “We believe with certainty, so 99 to 100 percent confidence, that the first item ignited in this blaze was a Christmas tree” on the second floor of the home, Thiel said at a news conference, adding that a lighter was found nearby. A five-year-old, who is one of just two survivors, was the only person on the second floor at the time the tree was ignited, he added. “We are left with the words of that 5-year-old child, that traumatized 5-year-old …