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DENVER — In Colorado Springs Tuesday night, some 800 people showed up at a fundraiser organized by the Independence Institute in support of its lawsuit challenging the state’s new gun control laws.

Even Douglas Bruce, the notoriously frugal author of Colorado’s Taxpayers Bill of Rights, was seen passing around a collections basket at the event, which reportedly raised close to $15,000.

But the lawsuit, which was filed earlier this month on behalf of 55 of Colorado’s 62 elected sheriffs, is mostly being funded not by grassroots donations but by the biggest Second Amendment rights group in the world.

The National Rifle Association.

Dave Kopel, the Independence Institute’s resident Second Amendment expert, is the lead attorney on the lawsuit.

And FOX31 Denver has found that Kopel has received $1.39 million in grant money from the N.R.A. Civil Rights Defense Fund between 2004 and 2011. The group continues to fund Kopel, although tax information from 2012 and 2013 isn’t yet available.

“They’re the oldest civil rights organization in the United States,” Kopel told FOX31 Denver this week. “I’m a well-known Second Amendment expert and researcher. It would make sense that they would support what I do.”

But Kopel has acknowledged the N.R.A.’s financial support before to reporters, but not when testifying earlier this year before Congress and before state lawmakers at the Capitol in Denver.

“I’ve never heard a think tank person testifying before the Congress reveal their organization’s funders,” Kopel said. “We don’t ever disclose anything publicly about our donors other than what the donors themselves disclose.

“Given that they’ve disclosed it, it seems legitimate for me to confirm that. I think every legislative body knows that think tanks are private, charitable organizations that raise money from people who agree with them.”

On May 17, the day Kopel stood in front of several Colorado sheriffs at a press conference announcing the suit at the Independence Institute’s Capitol Hill offices, Tom Mauser, whose son, Daniel, was killed in the 1999 Columbine High School massacre, called the think tank and asked whether the group took money from the N.R.A.

“I asked them if they got money from the N.R.A., and they wouldn’t tell me,” Mauser told FOX31 Denver. “They said, ‘look it up for yourself’.

“I think they’re hoping people won’t make the connection, because you have to dig a bit deep.”

Kopel disputed that account, telling FOX31 Denver that Mauser raised his voice at the woman who answered the phone after she told him that the Independence Institute isn’t obligated to reveal its donors but that the group’s 990 tax form is available online.

Given his decades of research on firearms and gun control policy, Kopel was invited to sit on the Senate floor as state lawmakers debated the package of Democratic gun control measures this March. At one point, he helped the sponsors of the magazine ban amend the proposal so that it didn’t ban all shotguns.

“Now that we know Kopel’s research is purchased with blood money from the gun manufacturers, it really calls into question its accuracy and its integrity,” said Laura Chapin, a consultant for the group, Ceasefire Colorado.

“After all, the NRA gets what it pays for.”

Kopel told FOX31 Denver that the lawsuit, which argues that the expanded background checks and the ban on high-capacity magazines violate the Second Amendment and the Americans with Disabilities Act, isn’t his alone.

“The lawsuit has a huge number of donors,” Kopel said. “I’m certainly not the only lawyer on this case. There are other lawyers being paid by other clients.”

And Kopel said the 55 Colorado sheriffs who have signed on to the suit as plaintiffs are doing so out of genuine concern about the new laws, which take effect July 1.

“They strongly believe that these bills endanger public safety,” Kopel said.

But Mauser, who has taken offense at those who have dismissed he and other relatives of gun violence victims as “props” of larger gun control groups, believes that the sheriffs are simply the public faces of a legal challenge that’s really coming from the N.R.A.

“I think it’s pretty clear who’s pulling the strings here,” Mauser said. “The polls are showing that people greatly support — over 80 percent in Colorado — support background checks. So why is there this resistance? Whose side are the sheriffs really on?”