(CNN) A photo of President Donald Trump's oldest son smiling while holding a semi-automatic rifle bearing medieval symbols and a cartoon of Hillary Clinton behind bars drew swift attention over the weekend, with hate group researchers pointing out that the medieval symbols have been embraced by extremist groups.

Donald Trump Jr. posted the photo Sunday on Instagram with a nod in the caption to the controversial design, which included a Crusader Cross -- also known as a Jerusalem Cross -- and helmet on the lower receiver, as well as a magazine featuring the image of the former secretary of state and Democratic presidential nominee.

"Nice day at the range. @rarebreedfirearms and @spikes_tactical adding a little extra awesome to my AR and that mag," Trump Jr. wrote, tagging the companies that design and sell the gun.

While symbols and references to the Crusades still hold religious and historical significance -- the Crusader Cross is included on the flag of the country Georgia -- far right groups have seized upon them, using them to represent an anti-Muslim ideology, according to the Anti-Defamation League and the Southern Poverty Law Center, two organizations that study hate groups.

A spokesman for Trump denied on Monday that the symbol on the gun, named by its manufacturer the Crusader Rifle, carried a white supremacist meaning and cited its presence on the Georgian flag and on a medal bestowed by the Pope.

"Symbols on firearms depicting various historical warriors are extremely common within the 2nd Amendment community. Don's Instagram post was strictly about him using a famous meme to mock Hillary Clinton, as he and many others have done on numerous occasions and will surely do again in the future, so long as it continues triggering humorless liberals," Trump spokesman Andy Surabian told CNN.

An avid hunter, Trump has posted other images of himself with weapons on his social media feeds. His affinity for controversial memes has helped bolster his own popularity among a Republican Party reshaped by his father.

History of the symbols

Symbols and references to the Crusades -- the Middle Ages campaign by Christian armies to reclaim the Muslim-controlled Holy Land -- have circulated for years inside the far right movement, making appearances in a manifesto written by a far-right gunman who killed dozens in Norway in 2011.

"The adoption of these symbols is meant largely as a way of signaling anti-Muslim sentiment in particular, but also this notion that Christianity needs to retake western civilization," said Howard Graves, a senior research analyst at the SPLC.

The gun companies that make and sell the Crusader rifle say it was inspired by history.

Rare Breed Firearms -- the manufacturer of the gun -- did not respond to a request for comment, but says on their website that the design was "inspired by some of the most fierce warriors who fought in nearly 200 years of epic conflicts known as the Crusades."

"This lower honors the warrior mindset. Technology evolves, warriors never change," the company wrote.

In an email, the CEO of Spike's Tactical -- the Florida company that sells the Crusader gun -- said that the gun and another AR with a Spartan helmet on it that they sell were "referencing famed historical soldiers" and are of a design that are "common among gun manufacturers, popular with gun owners throughout the country and have nothing to do with political ideology."

"It's objectively silly and dishonest for leftwing groups, like the SPLC, to claim that this symbol on our Crusader model has anything to do with hate or an extremist ideology. In other words, these people have no idea what they're talking about and should apologize for their outrageous smears," Cole Leleux, the CEO, said.

Spike's Tactical drew criticism in 2015 when they sold another AR model that a company spokesman told news outlets at the time was built to ensure it "would never be able to be used by Muslim terrorists to kill innocent people or advance their radical agenda."

That gun, also called the Crusader, featured an etching of a Bible verse as well as the Latin phrase "Deus Vult," another medieval term meaning "God wills it" that has recently become a rallying cry for white supremacists, according to hate group researchers.

Dan Zimmerman, the managing editor of The Truth About Guns, a website about firearms with a pro-gun leaning, told CNN that adorning guns with symbols is not common, but called it a "niche design that some people find attractive."

"There are all kinds of designs for AR lowers, from skulls to Sparta helmets," Zimmerman said.

A spokesman for the ADL, Jake Hyman, said the Sparta helmet symbol has also been co-opted by some right-wing extremists, and symbols like the Crusader Cross have recently been used to deface mosques in the US, according to Graves. The man accused of killing scores of Muslims at prayer in Christchurch, New Zealand, last year inscribed his weapons with references to the Crusades.

When white supremacists appeared in the Charlottesville march with shields bearing a red cross and the words "Deus Vult,"a coalition of Medieval scholars groups denounced what they called an "appropriation" of medieval symbols in a "fantasy of a pure, white Europe that bears no relationship to reality."

"As scholars of the medieval world we are disturbed by the use of a nostalgic but inaccurate myth of the Middle Ages by racist movements in the United States," the groups wrote.

White supremacist voices have gained prominence in recent years, with analysts like the ADL and SPLC pointing to the President's refusal to condemn racial violence by alt-right protesters at a 2017 rally in Charlottesville, Virginia, as emboldening the movement.

After wide blowback to his remarks on Charlottesville, Trump later called neo-Nazis, white supremacists and other hate groups "repugnant to everything we hold dear as Americans" and a month later, he signed a resolution condemning white supremacy.