As New York City prosecutors worked Thursday to bring murder charges against Luigi Mangione in the brazen killing of UnitedHealthcare CEO Brian Thompson, supporters of the suspect are donating tens of thousands of dollars for a defense fund established for him, leaving law enforcement officials worried Mangione is being turned into a martyr.

Several online defense funds have been created for Mangione by anonymous people, including one on the crowdfunding website GiveSendGo that as of Thursday afternoon had raised over $50,000.

The GiveSendGo defense fund for the 26-year-old Mangione was established by an anonymous group calling itself “The December 4th Legal Committee,” apparently in reference to the day Mangione allegedly ambushed and gunned down Thompson in Midtown Manhattan as the executive walked to his company’s shareholders conference at the New York Hilton hotel.

“We are not here to celebrate violence, but we do believe in the constitutional right to fair legal representation,” the anonymous group said in a statement.

The crowdfunding campaign prompted donations from more than 1,500 anonymous donors across the country, many of them leaving messages of support for Mangione, including one person who called themselves “A frustrated citizen” and thanked Mangione for “sparking the awareness and thought across this sleeping nation.”

The GiveSendGo fund for Mangione appeared to be briefly taken down before it was restored on Thursday.

In a statement to ABC News, a spokesperson for GiveSendGo said the company “operates with a principle of not preemptively determining guilt or innocence.”

“Our platform does not adjudicate legal matters or the validity of causes. Instead, we allow campaigns to remain live unless they violate the specific terms outlined in our Terms of Use. Importantly, we do allow campaigns for legal defense funds, as we believe everyone deserves the opportunity to access due process,” the GiveSendGo spokesperson said.

The spokesperson added, “We understand the concerns raised by such campaigns and take these matters seriously. When campaigns are reported, our team conducts a thorough review to ensure they comply with our policies. While other platforms may choose a different approach, GiveSendGo’s core value is to provide a space where all individuals, no matter their situation, can seek and receive support, with donors making their own informed decisions.”

Other crowdfunding sites such as GoFundMe have also taken down campaigns soliciting donations for Mangione’s defense.

“GoFundMe’s Terms of Service prohibit fundraisers for the legal defense of violent crimes,” the crowdfunding website said in a statement. “The fundraisers have been removed from our platform and all donors have been refunded.”

Amazon and Etsy have removed from their websites merchandise featuring Mangione, including T-shirts and tote bags reading “Free Luigi” and the phrase “Deny, Defend, Depose,” words police said were etched in the shell casings discovered at the scene of Thompson’s homicide.

“Celebrating this conduct is abhorrent to me. It’s deeply disturbing,” Manhattan District Attorney Alvin Bragg told ABC News senior investigative reporter Aaron Katersky in an interview Wednesday night. “And what I would say to members of the public, people who as you described are celebrating this and maybe contemplating other action, that we will be vigilant and we will hold people accountable. We are at the ready.”

Prosecutors at the Manhattan district attorney’s office have begun presenting evidence to a grand jury as they work to try to secure an indictment against Mangione, sources told ABC News on Thursday.

Mangione’s attorney, Thomas Dickey of Altoona, Pennsylvania, where the suspect was arrested Monday following a five-day manhunt, said his client is presumed innocent and will plead not guilty to any charges filed against him. Mangione is contesting extradition to New York.

Asked about people contributing to Mangione defense funds that have popped up, Dickey said, “People are entitled to their opinion and, like I said, if you’re an American and you believe in the American criminal justice system, you have to presume him to be innocent and none of us would want anything other than that if that were us in their shoes. So, I’m glad he had some support.”

In a later interview on CNN, Dickey said he is leaning toward not accepting any money from his client’s supporters for his defense.

“To be honest with you, I probably wouldn’t,” Dickey told CNN. “I just don’t feel comfortable about that. So, I don’t know. I haven’t given that much thought. Obviously, my client appreciates the support that he has, but I don’t know, it just doesn’t sit right with me, really.”

Retired FBI special agent Richard Frankel said that in previous politically-charged violent crimes, suspects have received unsolicited support.

“We saw it with the Unabomber,” said Frankel, an ABC News contributor, referring to Ted Kaczynski — the mathematician-turn-domestic terrorist who blamed technology for a decline of individual freedom and mailed handcrafted explosives to targeted individuals between 1978 and 1995.

Frankel said Eric Rudolph, who detonated a bomb in Atlanta’s Centennial Olympic Park during the 1996 Olympic Games and carried out three additional bombings as he eluded capture for five years, also attracted supporters.

“In my opinion, they’re supporting individuals who have committed potentially terrorist acts, but it’s a politically charged act,” Frankel said

Referring to the Thompson killing, Frankel added, “You can be up in arms about the healthcare industry, but you can’t threaten or actually hurt members of the healthcare industry.”

Most recently, Marine veteran Daniel Penny was acquitted of criminally negligent homicide in the chokehold death of Jordan Neely, a homeless man who was acting erratically on a New York City subway, after supporters donated more than $3 million to his legal defense fund.

Law enforcement officials have expressed concern that Mangione is being turned into a martyr. Someone this week pasted “wanted posters” outside the New York Stock Exchange naming other executives.

A bulletin released Wednesday by the Delaware Valley Intelligence Center, a multi-agency law enforcement intelligence-sharing network based in Philadelphia, included a photo of a banner hanging from an overpass reading, “Deny, Defend, Depose.”

“Many social media users have outright advocated for the continued killings of CEOs with some aiming to spread fear by posting ‘hit lists,'” the bulletin, obtained by ABC News, reads.

Meanwhile, New York Police Department investigators continue to build a murder case against Mangione, who is being held in Pennsylvania on charges stemming from his arrest there, including illegal possession of ghost gun and fraudulent identification. Mangione has pleaded not guilty to the charges in Pennsylvania.

On Wednesday, NYPD Commissioner Jessica Tisch said that the three shell casings recovered at the scene of Thompson’s shooting matched the gun found in Mangione possession when he was arrested. She also confirmed that Mangione’s fingerprints were recovered from a water bottle and the wrapper of a granola bar found near the crime scene.