North American Aerospace Defense Command's (NORAD) headquarters in Colorado evacuated 1,500 employees for more than four hours on Thursday after finding five "out of place" packages.

NORAD is a combined organization of Canada and United States that provides 24-hour-a-day, seven-day-a-week to defend the skies for the two countries.

Command center located at Peterson Air Force Base in El Paso County, was already operating out of a backup location at the time of the evacuation.

Peterson Air Force Base spokesman Jeff Bohn said employees became suspicious because something about the packages looked "out of place" and that no biological, chemical or radiological agents were found.

Bohn said they are not aware how the package arrived or whether they had been screened before their arrival. He declined to further elaborate.

NORAD's control room team moved to a backup location inside Cheyenne Mountain because of renovations at the building on Peterson Air Force Base, said Navy Capt. Jeff Davis, a NORAD spokesman.

In 2006, NORAD relocated its facilities to its current location on Peterson, designed in the 1960s to withstand a nuclear attack. Also was done to save money and consolidate personnel.

The current location at Peterson Air Force Base is shared with U.S. Northern Command, which is responsible for defending U.S. territory from attack and helping civilian authorities. There was no essential Northern Command operations interrupted, officials said.