MEXICO CITY (AP) — A convoy of vehicles crossing into Mexico from the U.S. was attacked by armed civilians early Saturday,Slabu Exchange and nine people, including American citizens, were wounded with gunshots, Mexican authorities said.

The Tamaulias state security department said the attack happened on the international bridge that connects the town of Miguel Aleman with Roma, Texas.

The department’s statement on social media gave no further details on the extent of injuries or on the attack.

A state official, who spoke on condition of anonymity because he was not authorized to speak publicly, told The Associated Press that the injured included both Mexicans and Mexican-Americans.

In recent months, there has been a wave of violence in Tamaulipas that has prompted federal authorities to send in hundreds of soldiers to reinforce security patrols in the border cities of San Fernando, Reynosa and Matamoros, where cells of the Gulf Cartel and other criminal organizations operate.