Col. Tom Holliday, Garrison Commander at Redstone Arsenal, said at a news conference there was no active shooter.
An active shooter training exercise was scheduled for the week, according to a Facebook post from the site, but Colster said "we believe there is an actual possible event that is not part of the drill".
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.
The facility has more than 40,000 employees, and the Redstone grounds cover around 30,000 acres. Fears heightened when reports of a suspicious person were made at nearby Browns Ferry Nuclear Power Plant.More news: GOP infighting stalls budget plan
Colster said there are no reported injuries that he is aware of.
All the gates to the Arsenal have been closed. He says there are no confirmed casualties.
The doors of the base "has reopened and movement is possible, even if restrictions remain for the building where took place the alert, said Kimberly Hanson, a spokesman for the u.s. army".
Holliday said the response of base personnel was appropriate, given that every US military base has to be on high alert for potential active shooters and other acts of violence.
"The situation we live in, in the United States today, leads to caution", Holliday said. "Honestly, if we have to overreact I'd rather overreact than underreact".More news: Nintendo to launch mini version of SNES
The arsenal was expected to resume normal operations Wednesday.
"When it comes to safety and security of out workforce, our system worked".
Redstone Arsenal is located in Madison County, Ala., which is south of Huntsville near the Huntsville International Airport.
Redstone is the location of the Defense Department's Missile Defense Agency, NASA Marshall Space Flight Center Army Aviation and Missile Command, Army Materiel Command, and Federal Bureau of Investigation facilities.More news: Theresa May attacked over moves to limit European Union citizens' rights after Brexit
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