San Andrés is a pre-Hispanic site of El Salvador, whose occupation began around the year 900 BC as an agricultural town in the valley of Zapotitán in the department of La Libertad. This early establishment was vacated by the year 250 because of the enormous eruption of the caldera of Lago Ilopango, and was occupied again in the 5th Century, along with many other sites in the valley of Zapotitán. Between 600 and 900 AD, San Andrés was the capital of a Mayan lordship with supremacy over the other establishments of Valle de Zapotitán.
In 1996, the Government of El Salvador inaugurated the Archaeological Park of San Andrés, where the visitor can climb the pyramids, see the indigo production area, and visit the site museum. (source)
Thank you Xiomara!