On 16 December, two Pathet Lao companies overran Phou Den Din, only east of Lima Site 85. Shortly afterwards, Hmong units recaptured the village. Toward the end of 1967, US controllers at Lima Site 85 directed F-4, F-105 and A-1 fighter-bombers based in Thailand and South Vietnam in air strikes against North Vietnamese and Pathet Lao formations that were massing around the US facility at Phou Pha Thi. A-26 Invaders were called in for night missions, targeting movements of the enemy ground troops on Route 6 and Route 19. On 14 January 1968, the situation in northeastern Laos continued to worsen, as an estimated four PAVN infantry battalions captured the Laotian government's stronghold at Nam Bac. Despite the growing threat from North Vietnamese forces, the US military was still not permitted to reinforce the installation on Phou Pha Thi Mountain due to political sensitivities. The defense of Lima Site 85 was assigned to two CIA paramilitary officers who led about 1,000 Hmong soldiers, with 200 men guarding the ridge line and the remaining 800 in the valley below. They were reinforced by a Thai Border Patrol Police battalion of 300 men. In the first week of 1968, the combined PAVN and Pathet Lao forces probed Royal Laos Army positions in the area by launching several artillery attacks. On 10 January, a Pathet Lao patrol was driven from the area by the Hmong soldiers. Fearing the explosives attached to their equipment could be detonated by incoming artillery rounds, US technicians dismantled the charges and threw them over the cliff. On 12 January, CIA spotters reported a four-aircraft formation flying in the direction of Lima Site 85. They were Soviet-made Antonov An-2 biplanes. Two aircraft continued towards Lima Site 85, while the others turned away. The Vietnam People's Air Force, in one of its few air attacks during the conflict, tried to destroy the radar at Lima Site 85. The An-2s flew over Phou Pha Thi, and their crewmen dropped 120 mm mortar shells through the aircraft's floor and then strafed their targets with 57 mm rockets mounted on the wing pods. As they repeatedly attacked the facility, ground fire heavily damaged one AN-2, and it crashed into a mountainside. By now, CIA officers and US controllers at Lima Site 85 had managed to contact an Air America helicopter, which was faster than the Soviet-made biplanes.