But the lockdown was lifted late Tuesday afternoon when investigators determined those reports were unfounded, Garrison Commander Col. Thomas Holliday Jr. said at a news conference. The 911 calls were "not credible information," he said.
Holliday said 911 received two calls from the Sparkman Center around 9 a.m., one saying shots were fired and the other saying someone saw a weapon.
At first much of the sprawling military base was locked down. An Army email sent to facility personnel and relatives shortly after 10 a.m. Tuesday said an active shooter was reported on the second floor of Building 5301, the Aviation and Missile Command
at the Sparkman Center complex.
By 1 p.m., the lockdown had been lifted except for the Sparkman Center. Holliday said it took hours to clear that building because of its size -- about 1.5 million square feet -- and the number of people who work there.
Investigators will try to find out what did happen, he said. Base security personnel were working with the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives.
Holliday said the response to the 911 calls was appropriate.
"When it comes to the safety and security of our workforce, our system worked," he said. "I'd rather they overreact than underreact."
An ambulance was dispatched because of heat problems and minor injuries related to the lockdown, Holliday said.
The base has called off a drill on how to respond to an active shooter because personnel learned "everything they need" on Tuesday, Holiday said.
About Redstone Arsenal
Redstone Arsenal is home to the Army's Material Command
and its Aviation and Missile Command
Redstone has been the center of the Army's missile and rocket programs since the 1950s, when it became the home for the contingent of German rocket scientists who surrendered to American forces at the end of World War II.
Civilian rocket research was moved to NASA's Marshall Space Flight Center in 1960.