CHARLESTON, S.C. (Reuters) - The FBI on Thursday said it was investigating a false threat that a “dirty bomb” was on a container ship at a terminal at the Port of Charleston in South Carolina.

The terminal was shut on Wednesday night and reopened early Thursday morning by the U.S. Coast Guard after law enforcement officials completed a scan of the container ship Maersk Memphis, the Coast Guard said.

The Federal Bureau of Investigation opened its investigation because it is a crime to make a false threat against a U.S.-flagged ship, said Donald Wood, a spokesman for the bureau in Columbia, South Carolina.

The Maersk Memphis is a U.S.-flagged ship, according to the Maersk Line website.

No arrests had been made as of Thursday afternoon, Wood said.

Wood would not comment on reports in local news media outlets that a phone caller reporting the threat had said that the information came from an online conspiracy theorist.

The Coast Guard said the “original reporting source of the threat” had been detained for questioning.

Law enforcement agents scanned four containers aboard the Maersk Memphis ship in the Wando Terminal after reports of a “potential threat” on Wednesday, the Coast Guard said.

A dirty bomb is a conventional explosive device designed to release radioactive material, contaminating the area around it.

Copenhagen-based Maersk Line, the world’s biggest container shipping company, said the Coast Guard had informed it of the threat of a dirty bomb aboard one of its vessels. It said all crew members were safe and ashore.

The Maersk Memphis, a 300-meter vessel, arrived in South Carolina from New York at about 7:30 p.m. Wednesday, according to Reuters data. About half an hour later, authorities were made aware of the potential threat and evacuated the terminal, the Coast Guard said.

By Thursday afternoon, the ship had left Charleston for Savannah, Georgia, according to Reuters data.