This story was updated at 5:46 p.m. ET.
The lockdown at NASA's Glenn Research Center in Cleveland this morning was caused by a glitch in an emergency telephone system, NASA has confirmed.
It "happened Friday when a message that was part of an agency Emergency Notification System test was inadvertently sent to an employee at Glenn," NASA headquarters spokeswoman Katherine Trinidad told SPACE.com.
A Glenn employee received an automated phone call warning that there was a shooter inside the building, the Associated Press reported. That call was meant for another employee, possibly at NASA's Kennedy Space Center in Florida, who had the same last name, the wire reported.
What was supposed to be merely a drill turned into a lockdown, with most employees and even the local police believing there was a gunman on the premises. Local news reported possible shots had been fired.
"We were shaking in our boots," said NASA Glenn spokeswoman Katherine Martin. "All we were told was there was a situation that involved a gun."
Local police and emergency responders arrived at the scene, while the center remained under lockdown for roughly an hour, Martin said.
"The Office of Protective Service officials at NASA Headquarters are conducting a review of the incident to ensure that this does not happen again," Trinidad said.
NASA declared the facility was "all clear" at about 10:50 a.m., and shortly after sent word that the event had just been a drill.
"We've had numerous drills but never one like this," Martin said.
The NASA center, named after former astronaut John Glenn, employs more than 3,400 people.
While many were traumatized by the experience, some saw the brighter side.
"It's a good practice if it ever happens," Martin said.