For General Motors, the mounting legal problems that began with a suit brought by a four-lawyer firm in Marietta, Ga., took a new turn Thursday, when two Am Law 100 firms with close ties to the company issued their verdict on what caused GM’s current recall crisis.

In a 315-page investigative report filed with the National Highway Traffic Safety Administration, GM’s lawyers from Jenner & Block and King & Spalding detail a “pattern of incompetence and neglect” at the automaker, but found no widespread conspiracy among top executives to cover up an ignition switch defect that has been linked to 13 deaths and sparked the recall of 2.6 million vehicles.