Four additional crew members were charged Wednesday in connection with a record-breaking 16-ton cocaine seizure aboard a cargo ship docked in the Port of Philadelphia, sources with knowledge of the investigation said.

Details of the roles the men allegedy played in the smuggling effort and the positions they held aboard the vessel remained under court seal while the investigation continues.

The sources, who were not authorized to speak publicly about the arrests, said all four — Bosco Markovic, 37; Alekandar Kavaja, 25; Nenad Ilic, 39; and Laauli Pulu, 32 — worked aboard the MSC Gayane, on which federal agents discovered the illicit cargo Monday in seven shipping containers. They remain in custody after making their first court appearance on Wednesday afternoon.

A spokesperson for the U.S. Attorney’s Office in Philadelphia declined to comment.

The additional arrests came a day after two other crew members — the ship’s second mate, Ivan Durasevic, and seaman Fonofaavae Tiasaga — were charged with violations of maritime drug smuggling laws.

Both men admitted to investigators that they helped haul aboard bales of cocaine from 14 smaller boats that approached the Gayane under cover of darkness at various points during journeys between Panama and the Peruvian coast in May and earlier this month. For their efforts, they said, they were promised payments of $50,000 or more.

Durasevic, 29, of Montenegro, and Tiasaga, 28, of Samoa, each implicated others aboard the vessel, including the Gayane’s chief officer, chief mate, an electrician, and an engineer, according to court filings that did not name those men and that have since been removed from the court’s public docket.

It was not clear whether any of those crew members was among the four charged Wednesday.

U.S. Attorney William M. McSwain described the seizure aboard the Gayane as the largest ever in the region and one of the biggest in U.S. history. He estimated the value of the drugs at more than $1 billion.

The Gayane remained moored in Philadelphia as investigators continued to examine its cargo. The vessel is owned by Mediterranean Shipping Co., a Geneva-based firm with operations in several U.S. cities.

Authorities said they hope to release more information later in the week.