Chicago officials were quick to blame heedless crowding for the three-decker porch collapse that killed 13 partygoers in the city's Lincoln Park district on June 29. "It is a tragic case of overloading the back porches," Chicago Fire Commissioner James Joyce told CNN. The Reuters news agency quoted Joyce as saying, "It appears to be a case of too many people in a small space." City Emergency Management Director Cortez Trotter told the press, "Common sense should prevail at some point." But in the following days, the city sued the building owner and the contractor who built the porch, charging that the porch was unsafe and was built without a permit. In turn, attorneys for the estates of people killed in the catastrophe have sued the city as well as the owners and builders, saying Chicago fell short of its duty to inspect properties and regulate construction. Suits focused immediately on undersized joists.

Seeking clues to the catastrophe's cause, attorneys and forensic experts are poring over dozens of blurry digital images released by the City of Chicago. The images show some of the construction details. At top left, the main posts away from the house are still in place, supporting the roof. At top right, a ledger board intended to carry joists...

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