The attempted sinking of the uss liberty?

why didnt president Johnson allow the 6th fleet to help even after they had intercepted messages from the israeli pilots who were attempting to sink the liberty...No matter who was shooting at our ship shouldnt Mcnamara who was over the military allowed them to defend the ship...why wasnt there an inveestigation into this matter for treason on the parts of those who refused to protect our countrymen that were being illegally attacked