A person who raised alarms about a possible "dirty bomb" on a container ship approaching the port of Charleston in South Carolina was being questioned Thursday after searches found nothing dangerous.

Coast Guard Lt. James Zorn said the Charleston office received a telephone call about a "dirty bomb" aboard the Maersk Memphis container ship around 8 p.m. Wednesday. The ship docked about a half-hour later.

Zorn said the Coast Guard contacted other federal, state and local agencies and set up a unified command that ordered the evacuation of the Wando Terminal of the South Carolina State Ports Authority in Mount Pleasant and set up a 1-mile safety zone around the dock on the Cooper River.

A "dirty bomb" is not a traditional nuclear weapon, but an explosive device containing radioactive or radiation-polluted material. By 3:30 a.m. Thursday, officials had scanned four suspect containers aboard the ship and determined there was no danger.

Zorn said the four "diplomatic" containers were searched based on intelligence gathered and information from the caller. He did not have other information on those containers, referring questions about them to the FBI.

FBI spokesman Don Wood in Columbia said he could not talk about the containers. He also would not provide details on the person being questioned or any possible charges.

The terminal was allowed to resume normal operations. No injuries were reported.