A suspicious package found Monday outside the MacDill Air Force Base visitor center in Florida contained what appeared to be an improvised explosive device, multiple sources told ABC News.

It is not immediately known if the device would have worked. The FBI said Wednesday that it is analyzing the suspicious package, but so far has declined to release details of the exact nature of what was found.

“Field screening was conducted on the contents of the package and identified possible energetic materials. Final lab analysis is not yet complete,” the FBI’s Tampa office said in an X post.

Military officials raised the threat level of the base to FPCON Charlie on Tuesday night, more than a day after the incident. FPCON Charlie is second highest Force Protection level out of five levels — indicating a serious threat has occurred and tightened security measures are in place around the base. 

On Wednesday afternoon, a shelter in place order was given after “a threat was made” against the base, according to officials.

“We take all threats seriously and are taking appropriate measures to prioritize the safety and security of our installation,” base officials said in a statement.

The shelter in place order was lifted less than an hour later.

MacDill Air Force Base is a critical command hub for the Pentagon, hosting U.S. Central Command — which directs military operations across the Middle East — as well as Special Operations Command.

Three of the six airmen who died in last week’s military refueling plane crash in Iraq were stationed there.

ABC News’ Cindy Smith contributed to this report.