Redstone Arsenal tweeted that the facility is on lockdown and people should "run hide fight".
Redstone Arsenal spokesman Christopher Colster said that he was not sure what to say.
Limestone County sheriff's spokesman Stephen Young said the reaction to the situation at Browns Ferry was likely "an abundance of caution-type thing based on what's going on elsewhere", referring to the arsenal.
Reports that there was a drill scheduled Wednesday added to the confusion surrounding the possible active shooter. There are not yet any confirmed casualties. Movement remains restricted in the area near the intersections and roads around the Sparkman Center.
Col. Holliday says it took time for investigators to clear the 1.5 million square foot Sparkman Center. "I'd rather they overreact than underreact". The size of the facility, which houses Army Aviation and Missile Command headquarters, forced officials to leave the lockdown in place for hours as they searched.
The governor's office says Alabama Gov. Kay Ivey is being updated about the situation, but her office couldn't provide any details.
According to al.com, Redstone Arsenal has more than 40,000 employees spread out over a 30,000 acre facility.
Normal operations resumed shortly before 1 p.m. "That was proven today".
'We do not believe there is a threat or we would not have opened the installation back up, ' he said.
Ambulances have been seen responding to the area.
The FBI said its agents headed to the scene.
The base is the army's centre for rockets and missiles and is also home to the NASA Marshall Space Flight Centre.
"We successfully mitigated a threat today with this arrest", said Roger Stanton, of the Federal Bureau of Investigation office in Birmingham.
The Marshall Space Flight Center sent an alert at 10:00 AM Tuesday warning that a "situation exists near the Sparkman Center, RSA bldg".
The commander of an Alabama military post says two 911 calls about a potential active shooter prompted a lockdown at the installation but no shooter was ever found and no shots were fired.