In 1966, construction of the World Trade Center began with a groundbreaking that razed 13 square blocks of low rise buildings, some of which predated the US Civil War. The construction was under the auspices of the semi-autonomous Port Authority of New York and New Jersey. In 1970, construction was completed on One World Trade Center, with its first tenants moving into the building in December 1970. Tenants first moved into Two World Trade Center in January 1972. The ribbon-cutting ceremony was on April 4, 1973. Ultimately the complex came to consist of seven buildings, but its most notable features were the main twin towers.
http://en.wikipedia.org/wiki/World_Trade_Cente r#Overview