Federal and local authorities evacuated a terminal at the Port of Charleston in South Carolina on Wednesday to investigate a "potential threat" in a container ship, the U.S. Coast Guard said.
"A 1 NM (nautical mile) safety zone has been established around the vessel while law enforcement authorities investigate the threat," the Coast Guard said on Twitter.
The actions were taken out of an abundance of caution after a "conspiracy theorist" made a claim about a threat aboard the Memphis on YouTube, Coast Guard Sector Charleston spokesman Lt. James Zorn told local WCIV television.
A man named George Webb claimed the Memphis was carrying a "dirty bomb" after it had been loaded in Pakistan during a recent trip, he said.
However, there was no credible information the threat was real, Zorn added.
The threat came at around 8 pm (1200 GMT Thursday), the Coast Guard said.
"Unified Command determines no existing threat to the port. Terminal will reopen and safety zone has been lifted," the U.S. Coast Guard Southeast said later on Twitter.
There were four containers where a threat was posed at the terminal, it said. "These have been scanned and cleared by authorities."