If you've followed the Rangers' quest for the Stanley Cup, then you've had a few options if you've wanted to watch.

You could buy a ticket, though the cheapest seats for playoff games at Madison Square Garden cost around $400.

You could watch on TV from home, which would require forking over about $100 a month for a cable package, depending on the plan.

Or you could watch the games online. For nothing.

It takes only a few minutes of surfing the Internet to find a live Rangers game courtesy of someone who is streaming the NBC Sports cable network and Mike Emrick's electrifying play-by-play for free.

These pirate sports sites—some of which even make money from advertising—are taking advantage of technology that's making it easier to live-stream high-definition TV feeds, enabling them to expand their audiences beyond diehard fans willing to put up with choppy video and erratic sound. In short, the same kind of bandits who attacked movies and music are now assaulting the multibillion-dollar world of sports.

"It's a huge, huge concern," said Austin Berglas, head of U.S. cyber investigations at Kroll and a former FBI special agent who established the bureau's financial cybercrimes task force.

"I did a lot of work with Major League Baseball on this," Mr. Berglas said, "and they lose a ton of money from piracy."

One reason sports teams are losing "a ton" of money is there's a ton of money to lose. Last year, $60 billion was paid out to professional sports teams in North America by fans, sponsors and media outlets, according to a report by PricewaterhouseCoopers. The broadcasting rights fees paid by giant publicly traded companies in New York, such as CBS, Fox, Time Warner Cable and Verizon Communications, jumped by more than 25% between 2012 and 2014, PwC reported. For those scoring at home, that's faster than Apple's 17% revenue growth during the same period.

Yet exactly how much money is leaking out via lost advertising, sponsorships and subscriptions is something that no one will own up to. Unlike the music and movie industries, the sports world has refused to publicly quantify its piracy problem.

Stealing broadcasts

Bob Bowman, chief executive of Major League Baseball Advanced Media, said that while he doesn't believe stolen broadcasts are costing his sport significant sums, baseball employs a crew of six to stop the folks who steal its programming.
"We go after copyright violators vigorously," Mr. Bowman said. "At the same time, we know technological advances have made piracy easier to do and we'll never catch everyone. In the end, we play whack-a-mole."

A few figures help illustrate the challenge. One of the most popular pirate sports sites, Stream2watch.me, attracts 389,000 monthly visitors in the U.S., according to online-audience tracker comScore. That's a drop in the bucket compared with the 100 million cable or satellite TV subscribers.

But the trouble is that dozens, if not hundreds, of bootleg sites pop up all the time. Sometimes, Mr. Bowman said, a site shut down by one country will pop up a few weeks later in another. Indeed, 54% of U.S. residents ages 18 to 34 admit to having watched pirated content of some kind, according to a survey of 1,115 of them last year by Iredeto, a computer-security firm.

The loss of any paying audience for sports is a problem for the likes of Time Warner Cable and other carriers fighting to keep customers from defecting to Netflix and other less expensive, legal sources of programming. These providers are in trouble if they lose control of their position as the only game in town for those who need to watch their favorite teams regularly.

"Sports are one of the few reasons why consumers are still paying for the cable or pay-TV bundle," said Amy Yong, an analyst at Macquarie Securities. A spokeswoman said Time Warner Cable "would agree" with Ms. Yong's observation.

Broadcasters and sports leagues for the most part don't wish to publicly discuss the issue.

An NBC Sports spokesman would say only, "We take this issue seriously and continue to be vigilant in our efforts to combat piracy." A National Hockey League spokesman said the Manhattan-based league "has been working actively for a number of years in cooperation with law enforcement, other sports leagues and related media organizations to enforce against the unauthorized live streaming of NHL games."

Behind their wall of silence are clear signs of growing alarm over digital bandits.
Last month, days before the big Floyd Mayweather-Manny Pacquiao fight, Time Warner's HBO and CBS' Showtime Networks filed a lawsuit alleging that a website was going to steal their broadcast. A federal judge ruled in favor of the networks, but many people watched for free anyway on live streaming from Periscope. Even so, HBO and Showtime reported that 4.4 million pay-per-view buys generated more than $400 million.

Although leagues and broadcasters lobby the government to crack down on piracy, there haven't been many criminal cases. The highest-profile action came in 2011, when federal prosecutors in Manhattan indicted a 32-year-old fellow from a Houston suburb for streaming sporting events and making $90,000 in ad revenue along the way. Ultimately, the feds let the defendant off the hook, consenting to a deferred-prosecution agreement in 2013, and six months later U.S. Attorney Preet Bharara agreed not to press the case further.

One reason the government has not been more aggressive: Although leagues and their broadcast partners are undeniably being harmed financially by piracy, the hefty prices of tickets and cable subscriptions don't make them the most sympathetic of defendants.

"These probably aren't the greatest cases to put in front of a jury," Mr. Berglas said.

Paid in Bitcoin

Fears of being prosecuted are so low that some pirates are busy making a business of their thievery. Some charge for subscriptions to illegal feeds and get paid in Bitcoin, a risky proposition because of the digital trail left behind. Most seem to prefer to swipe a page from the established broadcasters and stream enough soccer, hockey and other games so they build audiences large enough to attract advertisers.

That appears to be the approach taken by LiveTV.sx. The site, administered in Panama with a domain registered in the Caribbean by someone who evidently lives in Kazakhstan, attracts 165,000 monthly visitors in the U.S., according to comScore, and many more in Europe. It is the 1,750th-most-popular website in the world and is especially big in Russia, according to Alexa.com, a Web analytics company owned by Amazon.

LiveTV.sx streams dozens of pirated soccer, hockey, basketball and other broadcasts daily, many of which come with banner or pop-up ads. Based on the volume of streams and ads, Ted Davis, a digital-marketing expert at New York University, said the site may be generating "several million" dollars in net annual profit. One reason: low overhead.

"It looks like their cost of doing business is the subscription to a satellite network whose feeds they are streaming," Mr. Davis said. "Although it's entirely possible they paid someone off to give them the master key."