NEW: The lockdown is lifted shortly before 6 p.m., a spokeswoman says
The incident was at the expansive Savannah River Site in western South Carolina
Scans found possible "explosive residue" on a delivery truck, agency says
Authorities triggered an emergency response on Monday – including barring all incoming traffic – at a South Carolina nuclear site because of what was described as “a potential security event.”
The Savannah River Site, which is affiliated with the U.S. Department of Energy and National Nuclear Security Administration, announced the precautions via Facebook around 3:42 p.m.
About one and a half hours later, the site explained why: “Electronic and canine scans of a vendor delivery truck indicated a possibility of explosive residue on the truck.”
That hit spurred authorities from South Carolina and Georgia to join the site’s security contractor, Centerra, according to the Savannah River Site release.
The lockdown was lifted at 5:52 p.m., according to Savannah River Nuclear Solutions spokeswoman Lindsey MonBarren.
“An offsite law enforcement investigation has found no explosive residue or device on the truck,” the site explained on Facebook.
The Savannah River Site encompasses about 200,000 acres, the vast majority of it pine forest, in eastern South Carolina near the Georgia border south. It’s about 70 southwest of Columbia.
It produced about one-third of the United States’ weapons-grade plutonium and all its tritium, both of which are key components in nuclear weapons, during the Cold War.
As other nuclear sites closed down in recent decades, their and other materials were sent to the South Carolina site for safekeeping, according to its website.
In addition to safeguarding this material, work continues on the site in research and development, converting highly enriched uranium for use at commercial nuclear reactors, taking in more spent nuclear fuel and producing new tritium for national security purposes.
CNN’s Dave Alsup contributed to this report.