 Complete the following phrase:  During the post-war economic boom, a large wave of internal migration especially from rural areas of Southern Italy, moved to Milan. The population grew from 1.3 million in 1951 to 1.7 million in 1967. During this period, Milan was largely reconstructed, with the building of several innovative and modernist skyscrapers, such as the Torre Velasca and the Pirelli Tower. 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 