CIUDAD VICTORIA, Mexico—Gunmen in vehicles opened fire in the Mexican city of Reynosa on the U.S. border, killing at least 14 people and causing widespread panic, authorities said. Security forces killed four suspects, including one who died near a border bridge. The attacks began Saturday afternoon in several neighborhoods in eastern Reynosa, which borders the Texan city of McAllen, according to the Tamaulipas state agency that coordinates security forces. Images posted on social media showed bodies in the streets. Authorities did not immediately comment on a possible motive for the attacks. The area’s criminal activity has long been dominated by the Gulf Cartel, but there have been fractures within the group. Reynosa Mayor Maki Esther Ortiz Domínguez demanded an investigation and said city residents must be protected. The Mexican army, National Guard, state police and other agencies mobilized after the shootings. During the security sweep, authorities said they detained a …