The last foreign incursion came in 1914, on the eve of the First World War, when an American expedition captured and occupied Veracruz as a response to the Tampico Affair against the background of the Mexican Revolution; which threatened the regional oil industry in which Americans were heavily invested. After a short but bloody firefight the Americans captured the city, including San Juan de Ulúa. After seven months of U.S. occupation, the Americans departed and handed back the city to the Mexicans. The national legislature awarded the port and city of Veracruz the title of "Heroic" for the fourth time following this incident. A portion of San Juan de Ulúa also served several times as the presidential palace, housing presidents such as Benito Juárez and Venustiano Carranza. The citadel was also used as a prison, especially during the early 20th-century regime of President Porfirio Díaz. It was alleged by some sources that in order to prevent prisoners from escaping, sharks were placed into the waters surrounding the island, so that they would kill anyone attempting to escape .