A total of 32 tons of monomethylamine, a chemical used to produce synthetic drugs, were seized aboard a ship that arrived from China, the Mexican Navy Secretariat said.

The seizure was made on April 30 by marines, customs agents and Attorney General's Office personnel at the Gulf port of Veracruz, the secretariat said.

The manifest from the export firm identified the chemicals as aluminum sulfate, but tests confirmed it was monomethylamine.

The two containers carrying the chemicals were then seized from the MSC Brianna.

The chemicals were turned over to the AG's office "so it can investigate and determine the legal responsibility" for the shipment, the secretariat said.

The export firm was not identified and the secretariat did not say whether any arrests had been made.

Mexico's drug cartels are the main suppliers of illegal stimulants to the U.S. market.

The U.S. drug market has seen declining demand for cocaine and a simultaneous rise in demand for synthetic drugs, officials and analysts say.

Synthetic drugs are favored by users for their more prolonged effect and because they come in tablet form and are therefore considered "socially cleaner."