The signing of agreements with the Soviet Union and Czechoslovakia in mid-1960 was followed by a secret arms buildup in Cuba sponsored by the Soviets and their satellites. Early indications of that buildup came from signals intelligence -- the exploitation of Soviet and Soviet Bloc communications that carried information related to the arms supply operations. Beginning late in 1960 and extending through 1961, NSA intercepted messages concerning Soviet ships headed for Havana. The cargo manifests were suspiciously blank, indicating that what they carried was more than just palm oil or farm equipment.
For example, early in 1961 the Nikolay Burdenko arrived at the Cuban port of Mariel. The unloading operation was highly secret, but NSA caught Cubans discussing the arrival of tanks. It was hardly an innocent delivery.
As vessels poured forth their cargoes, the Cubans took extra precautions to keep them secret. They kept gawkers away from the wharf, unloaded at night, even prohibited overflights of the area while the unloading was taking place. Deliveries rolled away from the pier under black canvas and heavy guard. The United States government knew this from NSA intercepts of Cubans discussing the procedures.
The agreement with the Czech government in June of 1960 resulted in additional deliveries of small arms and ammunition, light aircraft, military vehicles, and equipment for military factories. Even more ominous was the training of Cuban pilots in Czechoslovakia. American intercept operators began to hear Spanish along with the usual mix of Slavic tongues coming from airfields in Czechoslovakia. As early as the fall of 1960, Cuban pilots were being trained in piston-engine trainers; later evidence showed that the training included jet aircraft. Thus it came as no surprise when, in mid-1961, CIA sources got wind of the imminent arrival of Soviet combat and transport aircraft, including the light bomber IL-28 and the state of the art MIG-15s, 17s and 19s. It was clear by then that a major upgrade of the Cuban air forces was in progress.
One problem shared by intelligence analysts and administration policymakers alike was the nature of the military deliveries. Most of the military equipment could be described accurately as defensive, but much of it could also be used in taking the offense. The primary questions for Americans was: would the Soviets introduce offensive missiles into Cuban territory?
Improvements to Cuban defensive capabilities continued to characterize Soviet deliveries. In May of 1961 NSA discovered references to shipborne radar, and, in June, radars associated with antiaircraft guns. Cubans chattered over the radio about "highly unusual aircraft" and impending training on various unspecified "Russian equipment."
Military deliveries increased in 1962, and along with them, Soviet attempts to conceal them. An NSA report in July indicated that Soviet vessels calling on Cuban ports were making false port declarations and declaring less than the known cargo carrying capacity. By late August NSA noted that there had already been fifty-seven voyages in a little over a month, and some ships were on their second voyage in that period of time.