Mexico's foreign ministry has accused a U.S. border patrol agent of fatally shooting a Mexican citizen over the weekend.
The shooting occurred near the bridge that connects Brownsville, Texas, with the Mexican town of Matamoros, the foreign ministry said Sunday in a statement condemning the violence.
The ministry said a Mexican national died after being shot by the U.S. agent, but did not release details about the circumstances surrounding the Saturday violence.
"The Mexican government has repeatedly expressed that the disproportionate use of lethal force in the exercising of immigration control functions is unacceptable under any circumstance," the foreign ministry said, calling for a thorough investigation.
A U.S. Customs and Border Protection spokesman said two agents fired shots Saturday morning during "dangerous encounters" along the border.
But U.S. officials have not confirmed any deaths related to the shootings.
"We have been unable to establish a connection between the shootings and a victim reportedly admitted to a Matamoros hospital," U.S. Customs and Border Protection spokesman Bill Brooks said.
Agents "were involved in two dangerous encounters" in the Rio Grande Valley Saturday morning, he said.
In one incident, an agent fired toward someone who was throwing rocks at him, he said.
"At approximately the same time, another nearby agent observed a person aiming a weapon at him and fired his service weapon in defense," he said.
The FBI is investigating, Brooks said.
In 2010, the FBI investigated after a border patrol agent shot and killed a Mexican teen along the border between El Paso, Texas, and Ciudad Juarez, Mexico. Federal officials closed the case in April, saying there was not enough evidence to pursue charges against the officer and that he acted within the agency's policies for use of force.
The officer received widespread condemnation from critics who questioned his use of deadly force against 15-year-old Sergio Adrian Hernandez Guereca, who authorities said had been throwing rocks at the agent when he was shot and killed.
Hernandez had a history of involvement with human smuggling and was on a list of repeat juvenile offenders, U.S. Customs and Border Protection spokesman Mark Qualia told CNN after the shooting.
An appeal is pending in a lawsuit filed by the teen's parents in U.S. federal court, accusing the officer of using excessive force. The Mexican government filed a friend-of-the-court brief in the case last week, noting that its request to extradite the border patrol agent for prosecution had been denied.
"As a matter of international responsibility, the fact that the victim was on Mexican soil when he was killed does not absolve the United States of responsibility for the acts of its agent," the brief said.