SAN JUAN, Puerto Rico -- Seven U.S. Postal Service workers in Puerto Rico have been indicted on charges they shipped thousands of parcels of heroin, cocaine and marijuana through the mail.

The Drug Enforcement Administration says the mail carriers are among 20 people charged in the case.

Puerto Rico is a favored transit point for drugs from South America, because packages that arrive in the U.S. territory do not have to clear customs to reach the American mainland.

Federal agents began executing arrest warrants for the defendants in pre-dawn raids around the San Juan area.

The DEA alleges the trafficking ring smuggled at least 100,000 pounds of marijuana worth more than $150 million into Puerto Rico over the last three years.

The mail carriers also are accused of facilitating the shipment of guns from the U.S. mainland to Puerto Rico.