There is little dispute that Sci-Hub, the website that provides free access to millions of proprietary academic papers, is illegal. Yet, despite being successfully sued twice by major American academic publishers for massive copyright infringement, the site continues to operate.

Some academics talk openly about their use of the repository -- a small number even publicly thank Sci-Hub founder Alexandra Elbakyan for her contribution to their research. Most academics who use the site, however, choose to do so discreetly, seemingly aware that drawing attention to their activities might be unwise.

Just how careful academics should be about using Sci-Hub has become a topic of concern in recent weeks, with many questioning whether sharing links to Sci-Hub could in itself be considered illegal.

1/12 So something not-fantastic happened yesterday.

I received an email from a lawyer at @twobirds, @moniquewadsted, on behalf of @ElsevierConnect regarding my blog post about where to download research papers and scientific articles for free. https://t.co/Bf3H19RZ14 -- Citationsy (@citationsy) July 31, 2019

The discussion started when the team behind Citationsy, a bibliography management tool based in Europe, tweeted that lawyers for Elsevier, a major publisher of academic journals, had threatened to pursue legal action if Citationsy did not remove a link to Sci-Hub from Citationsy's website. The link formed part of a blog post titled "Hacking Education: Download Research Papers and Scientific Articles for Free."

Citationsy's team removed the offending link. But they questioned whether the link was really illegal. They noted that there are plenty of links to Sci-Hub online, including on websites such as Wikipedia.

But does sharing information via links, as academics routinely do, really constitute complicity in Sci-Hub's flagrant disregard for the law?

Whether linking to materials that violate copyright law "is or is not a copyright violation" doesn't have a straightforward answer, said Martin Paul Eve, professor of literature, technology and publishing at Birkbeck, University of London.

"There are divergent legal views in different jurisdictions as to what constitutes infringement," he said.

Writing about the Citationsy news on his blog, Eve shared links to Sci-Hub he had found published within Elsevier's own journals.

"I would suggest that before throwing stones, Elsevier may wish to get its own glasshouse in order," he wrote.

Tom Reller, Elsevier's spokesman, said the links in question were in a very small number of articles.

"We've known about this for several months and had already started alerting authors to our efforts to change the links back to the version of record," he said. "We have implemented checks in our production process and are adding a few more to better prevent this."

Linking to infringing materials in the U.S. "is a bit of a gray area as far as specific case law is concerned," said Trotter Hardy, professor of law, emeritus, at the William and Mary School of Law in Virginia.

"It's pretty clear that linking to lawful, copyrighted material is not in itself an infringement," he said. "But it's less clear that linking to infringing material might itself be infringement."

It is usually considered unlawful for one person to support another's wrongdoing. In criminal law, this is known as "aiding and abetting," but these terms are not used in copyright law, said Hardy.

"We use 'contributory' and 'vicarious' infringement to mean more or less the same thing," he said.

Contributory infringement is found when a defendant "knows of the ultimate infringement (or should know) and does something that induces or materially contributes to that infringement," said Hardy. Vicarious infringement is found when a defendant "had the right and ability to supervise the ultimate infringer and stood to gain financially from the infringement."

In instances of academics linking to Sci-Hub, it is more likely to be a question of contributory infringement than vicarious infringement, said Hardy. So could linking to Sci-Hub be considered contributory infringement? Hardy thinks yes.

If a defendant knows that Sci-Hub, or material on it, is illegal, then they have the requisite knowledge. The question then becomes whether they "induced" or "materially contributed" to someone's infringement, said Hardy.

It's possible to argue that users would have found the infringing materials on their own. But linking to them does make it easier and quicker to find them -- something that Hardy would consider a "material" contribution to the infringement.

"Courts might differ regarding any of the above, of course, but that's my thinking."

Mitch Stoltz, senior staff attorney at the Electronic Frontier Foundation, sees the matter slightly differently. Laws in Europe, where Citationsy is based, are stricter than in the U.S. But here, it is "largely settled that linking to something is not copyright infringement," he said.

Links are the language of the internet, the way information is shared, and people shouldn't be punished for doing this, said Stoltz.

"If you're saying, 'Here's a site that lets you get a bunch of illegal, infringing materials for free, I really recommend you go here.' That's when you're encouraging people to infringe copyright," said Stoltz. "Providing the link in a way that doesn't show a preference is ok. And that's really important. The papers on Sci-Hub are not contraband. It's not like child pornography that is illegal to possess."

Kyle Courtney, copyright advisor at Harvard University, said he has received around 20 inquiries from colleagues about this issue in recent weeks. He said it had "not previously occurred" to him that linking to Sci-Hub might be considered illegal.

Courtney advises caution when dealing with the site in any capacity.

"Cracking down on links could be another way for publishers to try and trim the influence of Sci-Hub, which courts have generally agreed is illicit," he said.

Tomas Lipinski, dean of the school of information studies at the University of Wisconsin at Milwaukee, said that academics sharing links to Sci-Hub, should "be aware that much of the material on the site is suspect."

But he noted that not all content on the site is infringing. "It all depends on the contract between the author and the publisher," he said.

"Don't be afraid to link to things," said Lipinski. "But if you have a reasonable suspicion that the material you are linking to is infringing copyright, look for an alternative resource. Do the gut test -- If something doesn't seem right, don't use it."

Lipinski said it's generally wise to heed a notice to take down a link. But he also believes there is a risk that academics, who "generally want to do the right thing and are risk-averse," might "err too much on the side of caution," by unnecessarily removing content.

He and the other experts agree it's unlikely that publishers would pursue individuals for linking to Sci-Hub.

"Which publisher wants to be the one to sue a researcher for infringing?" asked Eve. "The optics of a multi-billion dollar company suing a researcher who gives them research material for free are terrible."