The fire that engulfed a social justice center in Tennessee a week ago, burning it to the ground and wiping out decades of valuable documents, led investigators to an "obscure" anti-Semitic symbol — one that also surfaced in last month's mass shooting in New Zealand, experts say.

Earlier this week, Jefferson County Sheriff Jeff Coffey described the black graffiti spray-painted in a parking lot near the Highlander Research and Education Center as a "hashtag symbol," according to The Knoxville News Sentinel.

"It's not a traditional, throw-it-in-your-face symbol that you would immediately recognize," he said.

Ash-Lee Woodard Henderson, the center's co-executive director, told NBC News on Friday that the large symbol had not been spray-painted on the parking lot before the March 29 fire, which was reported at about 5:30 a.m.

"I was there on Thursday, and no one from our staff recalled seeing it," she said. "When I did see it, I was like, 'What is it?'"

The News Sentinel published a picture of the symbol — identified by history scholars as belonging to the Iron Guard movement in Romania from the 1920s to ’40s.

"It's been rarely used and is a very obscure symbol," said Mark Pitcavage, a senior research fellow with the ADL Center on Extremism who studies hate iconography. "A lot of white supremacists wouldn't even recognize it."

The Jefferson County Sheriff's Office did not immediately respond to a request for comment Friday about the discovery or the latest on the investigation, but officials said last weekend that state bomb and arson agents were helping to determine if the fire was set intentionally. It's unclear when exactly the symbol would have been painted.

The sheriff's office said in a statement that it was investigating to "see if it has any affiliation to any individual or group."

The FBI would not confirm whether it is pursuing a hate crime investigation related to the fire.

WHAT DOES IT MEAN?

The Iron Guard is associated with a fascist, anti-Semitic movement that took off before World War II, Pitcavage said. It became one of the largest parties in Romania, supporting ultra-nationalism and anti-Jewish legislation, while aligning itself with members of the Romanian Orthodox Church.

Pitcavage said the symbol promoted by the movement represents jail bars, and its founder had been imprisoned before his death.

"The Iron Guard did not survive World War II, but in the decades after, white supremacists in Europe and other places resurrected it and repurposed them as more generic symbols and white supremacist symbols," he added.

HAS THERE BEEN A RESURGENCE?

While the symbol did not gain the same prominence as other pre-World War II-era iconography, Pitcavage said it was used in more recent years by members of the now-defunct Traditionalist Worker Party, which the Southern Poverty Law Center designated as a neo-Nazi group.

According to the News Sentinel, the group was active in the Knoxville area. Its leader, Matthew Heimbach, of Indiana, was photographed wearing a T-shirt bearing the Iron Guard symbol and its founder after the deadly Unite the Right rally in Charlottesville, Virginia, in 2017.

The Traditionalist Worker Party also took part in a counterdemonstration at a women's march in Knoxville in January 2018, the News Sentinel reported, adding that the group had about 20 members show up, while the march drew more than 14,000 supporters.

That same month in Knoxville, a nearly 100-ton dolomite stone nicknamed The Rock at the University of Tennessee's campus was found spray-painted with the letters "TWP" and various iconography, including the Iron Guard symbol.

The Rock at the University of Tennessee; photo taken by UT professor on January 22, 2018: pic.twitter.com/8hpzblEBKS — Lee Sessions (@LeeSessions77) January 22, 2018

The Rock was again vandalized last fall with swastikas.

The Southern Poverty Law Center, which has tracked more than 1,600 extremist groups in the United States, said it found 36 hate groups operating in Tennessee last year, including dozens associated with the white supremacist movement. The Traditionalist Worker Party, or TWP, ceased operating in early 2018.

Keegan Hankes, a senior research analyst with the law center, said former members would still be familiar with the symbol. Some of those members in eastern Tennessee have since joined a smaller offshoot group, he added.

"This graffiti is not as commonplace as the swastika," Hankes said. "It's a very specific symbol with a very esoteric place in the larger fascist worldview. However, whoever put it there is very well versed in these ideologies."

The Highlander center moved to its current location in New Market, east of Knoxville, in 1972. As the investigation continues, the potential that a fire was purposefully set there is not lost on Woodard Henderson.

While there hadn't been any recent threats, she said, the nonprofit — which has hosted civil rights leaders and social activists, including Martin Luther King Jr., Rosa Parks and Eleanor Roosevelt — has a meaningful place in history.

"We've spent nearly 90 years bringing people together, by race, religious practice, geography, class — and our people are resilient and so committed to this work," Woodard Henderson added. "We faced many storms similar to this one. This most recent one is devastating."

A CONNECTION TO NEW ZEALAND?

The Iron Guard symbol was discovered on a gun believed to have been used by the man accused of the March 15 attack at two mosques in Christchurch, New Zealand, that left 50 dead and 49 wounded.

The weapons and ammunition were marked with white writings that include anti-immigration messages, images shared online show. Social media posts apparently written by the suspect also contain anti-Muslim rhetoric, far right and far left political sentiment, and references to "radicalization" via the internet.

"The fact that the Traditional Worker Party, the (suspected) Christchurch shooter, and now this happening in Tennessee — it's more of a use of the symbol than it's seen in a long time," Pitcavage said.