Unable to take the city, the PAVN kept it under constant artillery fire. They also moved in more anti-aircraft guns to prevent aerial resupply. Heavy anti-aircraft fire kept RVNAF helicopters from getting into the city after April 12. In response, fixed wing RVNAF aircraft (C-123s and C-119s) made attempts, but after suffering losses, the U.S. Air Force took over on April 19. The US used C-130s to parachute in supplies, but many missed the defenders and several aircraft were shot down or damaged. Low altitude drops during day and night did not do the job, so by May 2, the USAF began using High Altitude Low Opening (HALO) techniques. With far greater success, this method of resupply was utilized until June 25, when the siege was lifted and aircraft could land at An Lộc. Over the entire course of the resupply effort, the garrison recovered several thousand tons of supplies—the only supplies it received during the siege.