The U.S. Embassy in Saigon sent its last message:
PLAN TO CLOSE MISSION AT ABOUT 0430 30 APRIL LOCAL TIME. DUE TO NECESSITY TO DESTROY COMMO GEAR THIS IS THE LAST MESSAGE FROM EMBASSY SAIGON.
The U.S. Ambassador, Graham Martin, initiated Operation Frequent Wind, the helicopter evacuation of the last remaining U.S. citizens and 'at-risk' Vietnamese allies from Saigon by U.S. Navy Task Force 76.
At 04:58, Ambassador Martin boarded a Marine CH-46 Sea Knight helo and was flown to the USS Blue Ridge. At 07:53, the final helo in the airlift extracted eleven Marine Security Guards from the embassy rooftop, and the evacuation was over.
See Tears Before the Rain for an excellent collection of first-person accounts of those last days.