At least 12 people died and dozens more were injured when a stampede broke out inside a fútbol stadium in El Salvador on Saturday (May 20).

According to Reuters, the incident happened at Cuscatlán Stadium in the capital city of San Salvador during a quarter-finals match between Alianza FC and Club Deportivo FAS. Cuscatlán Stadium is one of the largest stadiums in Central America with a capacity of approximately 44,000 fans.

“The Salvadoran Football Federation deeply regrets the events that occurred at the Cuscatlan Stadium,” the organization posted on Twitter. “It also expresses solidarity with the relatives of those affected and deceased in this incident.”

According to the New York Times, it is not immediately known what caused the stampede. Videos circulating online this weekend showed people pushing through an access gate at the stadium. There are also multiple videos and photos of injured people on the field.

“On behalf of the U.S. Soccer Federation and everyone in our soccer community, we extend our deepest condolences to all those impacted by the tragic events at the Estadio Cuscatlán in El Salvador last night,” U.S. Soccer tweeted Sunday (May 21). “We stand with our colleagues and friends at the Federación Salvadoreña de Fútbol during this time of mourning and offer our comfort and support to those Salvadoran people.

Salvadoran President Nayib Bukele said an investigation would take place and hold people accountable.

The Fédération Internationale De Football Association (FIFA) and the Confederation of North, Central America and Caribbean Association Football (CONCACAF) also extended their condolences to the families and friends of the victims.

“The safety of participants and fans attending football matches is of utmost importance,” CONCACAF said in a statement. “The Confederation will fully support all efforts to clarify what happened and to implement measures to prevent such incidents in the future.”