|Barrio||San Juan Antiguo|
Puerto Rico was ceded by Spain in the aftermath of the Spanish–American War under the terms of the Treaty of Paris of 1898 and became an unincorporated territory of the United States. In 1899, the United States Department of War conducted a census of Puerto Rico finding that the population of Ballajá was 1,217.
- Joseph Prentiss Sanger; Henry Gannett; Walter Francis Willcox (1900). Informe sobre el censo de Puerto Rico, 1899, United States. War Dept. Porto Rico Census Office (in Spanish). Imprenta del gobierno. p. 162.
- U.S. Geological Survey Geographic Names Information System: Ballajá Subbarrio
- Video of the Ballajá barracks in 2020