After the failure of the assault on 21 April, the battle devolved into a siege, with the North Vietnamese pounding An Lộc and its defenders with 1,200 to 2,000 mortar, rocket, and artillery rounds per day. An Lộc was completely surrounded and could only be resupplied by air, a situation made more difficult by the loss of the airfield. Resupply was accomplished, however, by 448 aerial missions which managed to deliver 2,693 tons of air-dropped food, medical supplies, and ammunition, supported by the US Army's "549th QM (AD)" (QuarterMaster, Air Delivery) Rigger Company, which had been rapid-deployed from Okinawa in early April.