Once the negotiations were successfully concluded, production lines were established at the Rogožarski plant in Belgrade and the Zmaj factory in nearby Zemun. The two plants were expected to build forty and sixty of the aircraft respectively, at a rate of twelve per month. Of the locally built aircraft, only twenty were completed by Zmaj; the Rogožarski plant did not produce any. A total of 44 aircraft were put into service with the VVKJ. Once in service, Hurricane Mk Is were used to equip the 52nd Fighter Group of the 2nd Fighter Regiment based at Knić, and the 33rd and 34th Fighter Groups of the 4th Fighter Regiment based at Bosanski Aleksandrovac. Hurricanes were also operated by the Independent Fighter Squadron of the 81st Bomber Group and by the Air Training School, both based at Mostar. All of these aircraft were deployed in the fighter/interceptor role. Immediately prior to the German-led Axis invasion of Yugoslavia in April 1941, 41 of the original 44 Hurricanes were serviceable. They were allocated as follows: At 06:45 on 6 April 1941, the "Luftwaffe" launched Operation Retribution, a series of concerted bombing attacks on Belgrade that coincided with air and ground attacks throughout the country. Several waves of German aircraft approached Belgrade during the day, initially Junkers Ju 87 "Stuka" dive-bombers escorted by fighters. About 08:00, Hurricanes of the 52nd Fighter Group engaged the second wave as it departed after bombing the city; one of the dive-bombers was shot down by three pilots from the 163rd Squadron. For the remainder of the first day of the invasion, the Hurricanes of the VVKJ saw little action, despite constant patrolling between Čačak, Kraljevo, and Kragujevac. Few of the aircraft had radio sets, so the fighters usually arrived too late to take part in the fighting. Two 4th Fighter Regiment machines were tasked with escorting Bristol Blenheim Mk I light bombers to attack targets in Austria, but they lost sight of the bombers in cloud cover. One of the pilots then attempted to intercept some German Messerschmitt Bf 109 fighters. The following day, the 2nd Fighter Regiment continued to patrol over central Serbia, protecting factories at Kraljevo and Kragujevac from potential German air attacks that never occurred. The 4th Fighter Regiment was also active, patrolling over Bosnia and Croatia, but saw little action except for attempts to intercept German reconnaissance aircraft.