We do not know what message the US Embassy in Dhaka had sent to the State Department after the Pilkhana massacre. We will have to wait for another 30 years to get full text of the official message from the US Embassy.
However, I can share with you the message the US Embassy in Dhaka had sent after the August 1975 coup. This declassified document confirms Sheikh Mujib was warned by the US about the coup but he simply brushed the information aside.
The Embassy also provided early analysis of the August 15 coup against President Mujibur Rahman. It tentatively predicted that the United States would enjoy greater influence under the government of new president Khondakar Mushtaque Ahmed.
The Embassy had also provided a narrative account and analysis of the military unrest and resulting coup of November 3–7, 1975.