FORT LAUDERDALE, Florida: The FBI said on March 18 that a suspicious package found outside a gate at MacDill Air Force Base in Tampa earlier this week contained "possible energetic materials."

The FBI Tampa office said in a statement posted on social media in the afternoon that an analysis of the material has not been completed. It said the investigation was continuing and provided no further details.

Energetic materials store large amounts of chemical energy and can include explosives, fuels, and propellants. The FBI did not reply to an email asking what kind of material was found in a package on March 16 outside the visitor center at MacDill Air Force Base.

Earlier, the base had issued a shelter-in-place order after receiving a threat. Officials said they take all threats seriously and are taking steps to keep the base safe, but they would not share details about the security measures.

The order was lifted about two hours later, but officials said the base is still on high alert.

On March 17, the base moved to Force Protection Charlie, or "FPCON CHARLIE," which is the second-highest military security level. This allows officials to put extra security measures in place as a precaution.

Personnel at the base were told to stay alert, follow instructions from security teams, and report anything suspicious right away.

United States Central Command (CENTCOM), which is based at MacDill, oversees U.S. military operations in the Middle East, Central Asia, and parts of South Asia.

MacDill is one of several U.S. bases on increased alert since the conflict involving Iran began.

Last week, all six crew members of a KC-135 refueling aircraft died in a crash while supporting operations related to Iran. Officials said three of them were linked to the 6th Air Refueling Wing at MacDill.