The website of TV Azteca, one of Mexico’s major television networks, had a video of the scene that showed panic-stricken adults and children still in swimwear – including pool floats – at the scene after the shooting.
The attack happened on the last day of the spring school break.
The resort adjacent to the university campus was mobbed by army and state police following the attack.
Guanajuato, a thriving industrial state, has become one of Mexico’s most violent states due to a dispute between the Santa Rosa de Lima crime group and the Jalisco New Generation Cartel, which is dedicated to fuel theft and drug trafficking.
Mexico has recorded more than 350,000 murders since the government controversially deployed the military to fight drug cartels in 2006, most of which are blamed on criminal gangs.