Huntsville Police helping out with possible active shooter at Redstone Arsenal.
There were no confirmed injuries or arrests, but officials said there would be an investigation into the incident.
The arsenal had planned active shooter training, but Redstone officials said Tuesday's lockdown is not a drill.
Alabama governor Kay Ivey's office couldn't provide any details.
Many Redstone offices do not allow employees to carry cell phones and communications are restricted in most areas.
The base is home to military units including the U.S. Army Aviation and Missile Command and elements of the Defense Intelligence Agency and the Missile Defense Agency.
Reports say the shooter could be at Building 5301, the aviation and missile command building at the Sparkman Center, according to CNN.
The active shooter drill schedule for this week has been canceled.
A photo taken from the Sparkman Center at the Redstone Arsenal Army base Tuesday June 27,2017 shows people evacuating the area.
"We believe that we have learned everything we needed to as part of the event that occurred" Tuesday, Holliday said. By 11:28am local time officials said checks on the base's childcare center found all children were safely locked inside.
Holliday said this is an isolated incident and that the initial callers claimed they heard shots and saw a possible gunman.
The Redstone Arsenal base's Facebook page posted an update that only one building at the base is locked down, but beforehand it issued a standard "run, hide, fight" warning about a possible shooter. It was later confirmed by Holliday the 911 calls came from the Sparkman complex. "I will continue to closely monitor and pray for a peaceful and quick resolution", she said.
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.