MIAMI (AP) – The Port of Miami was hit by its second terrorism scare in two days Monday when a package that was to be loaded onto a cruise ship tested positive for plastic explosives. Authorities later determined it was just a box of sprinkler parts.

The package was initially tested six times, and each time it came back positive for the military-grade explosive known as C4, the Coast Guard said.

The package was destroyed, and a Miami-Dade County police bomb squad determined the sprinkler parts contained a substance that “closely resembles” plastic explosives, said police spokesman Bobby Williams.

Williams said the instruments used to test the package sometimes give false positives.

“We still need to check it out,” Williams said.

The package was included in provisions that were to be loaded aboard Royal Caribbean International’s Majesty of the Seas. Explosives detection instruments gave the positive reading about 2 p.m., the cruise line said.

The FBI was monitoring the situation, spokeswoman Judy Orihuela said.

The developments came a day after three Middle Eastern men in a cargo truck sparked a brief terrorism scare at the port. After a bomb squad search, authorities concluded that the men were carrying automotive parts and that the scare stemmed from miscommunication. A judge dropped all charges against the three men Monday.

The Port of Miami is among the nation’s busiest. More than 3.6 million cruise ship passengers traveled through it in 2005, making it the world’s top terminal for cruise vacationers.