DONNA — After being processed by U.S. authorities, 33 Mexican soldiers who crossed the international boundary in four military Humvees on Tuesday afternoon were allowed to return to Mexico.

U.S. officials who interviewed the soldiers about the incursion on the Donna-Rio Bravo bridge could not divulge a reason for it — other than to say that the soldiers had lost sight of the line and crossed inadvertently.

But a source outside of law enforcement and with knowledge of border activity said the soldiers were chasing a South Texas resident who had crossed into Mexico, and although the soldiers did not capture the individual, U.S. authorities did.

That story could not be corroborated with U.S. authorities.

About 1:30 p.m., the four green Humvees on the Donna bridge “inadvertently” crossed the boundary into the United States, said Felix Garza, U.S. Customs and Border Protection spokesman.

“Apparently the men lost sight of the boundary and went across,” Garza said, adding that no one was detained and no charges were filed.

“The soldiers were performing their regular duties when they lost sight of the line,” he said.

After the soldiers crossed into the U.S., CBP officers at the bridge activated a series of protocols to respond to such incidents, Garza said, adding that all Mexican personnel were processed based on those protocols.

Shortly after 4 p.m., the soldiers were seen being loaded back into their vehicles and escorted back toward Mexico.

Bridge traffic was stopped temporarily as a precautionary measure when the soldiers crossed the line and when the soldiers were sent back to Mexico, Garza said. U.S. Immigration and Customs Enforcement agents at the bridge interviewed the Mexican soldiers in order to establish the reason for the incursion.

More information as to why the Mexican soldiers crossed the bridge was not readily available Wednesday.