MIAMI (CNN) -- Federal authorities have seized more than 5,000 pounds (2,250 kilograms) of cocaine, worth nearly $40 million, from a Panamanian ship docked in Miami.

Thirteen people, all from outside the United States, were arrested.

The drugs were seized Tuesday night aboard the cargo vessel Isamar, which was docked in the Miami River. The Isamar was carrying 35 cargo containers, but only two of those containers held any cargo. One held general freight; the other, cocaine.

U.S. Customs Service agents found kilo-sized bricks of cocaine in large plastic bags only slightly concealed behind propane gas tanks.

"We've been investigating this boat and its activities for a year. As it would come and go, we would watch it. This time when it tied up, we caught them," said Customs Service spokesman Michael Sheehan.

"We are seeing a return to the smuggling of the good old days, where they would move big shipments," Sheehan said. "They are getting gutsy again."

He said smugglers were returning to their old routes, because they "are tired of going through Mexico, where they lose half of their shipment."