During the post-war economic boom, the reconstruction effort and the so-called Italian economic miracle attracted a large wave of internal migration (especially from rural areas of southern Italy) to Milan. The population grew from 1.3 million in 1951 to 1.7 million in 1967. During this period, Milan was rapidly rebuilt, with the construction of several innovative and modernist skyscrapers, such as the Torre Velasca and the Pirelli Tower, that soon became the symbols of this new era of prosperity. The economic prosperity was, however, overshadowed in the late 1960s and early 1970s during the so-called Years of Lead, when Milan witnessed an unprecedented wave of street violence, labour strikes and political terrorism. The apex of this period of turmoil occurred on 12 December 1969, when a bomb exploded at the National Agrarian Bank in Piazza Fontana, killing 17 people and injuring 88.