[Source: Reuters]

Mexican and U.S. authorities said on Monday they were working to locate four Americans.

They were shot at by gunmen in northern Mexico and then kidnapped shortly after crossing the border.

The four unidentified Americans were in a white minivan with North Carolina license plates when they entered Matamoros, Tamaulipas, on Friday, the U.S. Embassy in Mexico said, seeking the public’s help in identifying the kidnappers.

The armed men fired on the passengers shortly after their vehicle crossed into Mexico and then herded them into another vehicle before fleeing the scene, the embassy said.

The U.S. State Department has advised Americans not to travel to Tamaulipas, in part due to the threat of kidnapping. The city of Matamoros is over the border from Brownsville, Texas,

A Mexican official told Reuters three men and one woman were kidnapped.

An innocent Mexican died in the incident, U.S. Ambassador to Mexico Ken Salazar said in a statement.

Mexico’s president said the Americans had crossed into Mexico to buy medication.

“I believe it will be resolved, I hope so,” President Andres Manuel Lopez Obrador said in a regular news conference on Monday, adding that Mexico’s public security ministry was working with the FBI to locate the missing U.S. citizens.

Speaking at a regular news briefing in Washington, White House spokesperson Karine Jean-Pierre said U.S. law enforcement was in touch with Mexican authorities, as were the departments of State and Homeland Security.

“Our thoughts are with the families of these individuals and we stand ready to provide all appropriate consular assistance,” she said. “We will continue to coordinate with Mexico and push them to bring those responsible to justice.”

The FBI is offering a $50,000 reward for the return of the four and the arrest of those involved.