By BERNARD CONDONThe parents of two college students killed in a Tesla crash say they were trapped in the car as it burst into flames because of a design flaw that made it nearly impossible for them to open the doors, according to lawsuits filed Thursday. The new legal threats to Tesla filed in Alameda County Superior Court come just weeks after federal regulators opened an investigation into complaints by Tesla drivers of problems with stuck doors. A fourth passenger was pulled from the car after a rescuer broke a window and reached in. Tesla doors have been at the center of several crash cases because the battery powering the unlocking mechanism can be destroyed in a fire and the manual releases that override that system are difficult to find. The lawsuit follows several others that have claimed various safety problems with Tesla cars.