(Reuters) – A lockdown at a U.S. Army base in northern Alabama on Tuesday was lifted after two telephone calls reporting an active shooter at the sprawling facility near Huntsville turned out to be false.

No threat was found and an investigation is under way to determine who made the calls from within a building where the shooter was reported to have been, Colonel Thomas Holliday, commander of the Redstone Arsenal, said at a press conference.

“We have no evidence of shots actually (being) fired,” Holliday said.

One of the calls to 911 emergency dispatchers said gun shots had been heard, and another said a gun had been seen. Neither of those were credible, Holliday said.

There were no casualties, and no one was arrested, Holliday said. There were no visitors on that part of the base on Tuesday, he added.

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. It also hosts the National Aeronautics and Space Administration’s Marshall Space Flight Center, where rocket research and testing is conducted.

A total of about 43,000 military, NASA and civilian personnel work at Redstone Arsenal.

Holliday said the response of base personnel was appropriate, given that every U.S. military base has to be on high alert for potential active shooters and other acts of violence.

It took hours to ensure that everyone was safe in the 1.5-million-square-foot (457,200-square-meter) Sparkman Center complex where the shooter was reported.

A full base lockdown lasted for about two-and-a-half hours, and some buildings were shut for four-and-a-half hours, Holliday said.

A drill to handle a situation similar to the one reported on Tuesday was to occur later this week, but was canceled after this incident, Holliday said.

(Reporting by Bernie Woodall in Fort Lauderdale, Fla. and Gina Cherelus in New York; Additional reporting by Timothy Mclaughlin in Chicago; Editing by Tom Brown and Dan Grebler)