LYNN – City inspectors ordered the owners of 144 South St. to demolish the six-family residence severely damaged Saturday by one of the year’s biggest fires.Thirty tenants, including 18 children, fled the afternoon blaze without injury leaving firefighters to wonder how the fire burned for a period of time before a third-floor tenant noticed smoke wafting from behind her stove.”To get that type of black smoke tells us the fire had a pretty good start,” said fire investigator Donald Baron.Baron on Monday said he hopes to tell Acting Fire Chief James Carritte today how the fire started. He spent last weekend and Monday interviewing residents of the first-floor rear apartment where investigators believe the blaze started in a closet.”We think we have a handle on how it started but we are not there yet,” Baron said.The fire and the volume of water required to extinguish it collapsed 144 South’s roof and filled its basement with water. Chief Building Inspector Roger Ennis sent owners Paul and Beata Pantzer of Marblehead a letter Monday declaring the building unsafe and ordering them to secure the property.”The structural integrity of the building has been compromised by the fire,” Ennis said, adding the Pantzers will probably consult with a structural engineer before preparing a plan outlining for the city how the building will be razed.Tenants aided by the Red Cross sought temporary residences in the Peabody Holiday Inn or with relatives.Fire crews from other communities provided mutual aid to Lynn crews while local firefighters battled the blaze.The fire rivaled two fires in January, including one on Lewis Street that destroyed ground-floor stores and left tenants homeless.