Mexican officials issued a vigilance alert Tuesday after thousands of dead fish washed up on the coast of the Gulf of Mexico.
Veracruz state governor Fidel Herrera issued the alert after fishermen reported dead fish along several kilometers (miles) of beaches off eastern Mexico.
"We have to determine if it was a natural phenomenon or if it is more generalized," Herrera told journalists, adding that Mexican Marines had taken samples from other coastal areas.
Investigators were studying the fish to determine the cause of the deaths, said Aurora Parissi, director of Veracruz state public health laboratory.
They were also studying possible links to dead fish which appeared last month on beaches in the Mexican state of Tamaulipas and the US state of Texas, both also on the Gulf of Mexico.