NEW YORK - The aggressive push federal prosecutors are making against potential insider trading is sending investigators into a legal gray area that may redefine insider trading itself.

Mutual fund company Janus Capital Group said Tuesday it was cooperating with an inquiry on insider trading. A day earlier, the FBI searched the offices of three hedge funds in New York, Connecticut and Massachusetts as part of what outside experts say could turn out to be one of the largest probes in Wall Street history.

Investigators are thought to be pursuing suspicions of trading by hedge funds and mutual funds that might have profited illegally using inside information not available to ordinary investors.

But the behavior being targeted by investigations appears more elusive and complex than the common understanding of insider trading - high-powered executives picking up the phone, whispering secret tips about big deals, and trading stock at an unfair advantage. These were the types of cases that ensnared executives such as Ivan Boesky in the 1980s and Enron's Jeffrey Skilling more recently.

How the alleged scheme may have worked is unclear. Federal prosecutors declined to comment on Tuesday. But what is clear is that the way information is shuttled around the world of finance today is faster, more complex and more shadowy. It involves an increasingly digitized financial world where networks of insiders can share bits of information with blinding speed.

It can also tap into so-called expert networks of industry analysts, experts and consultants who squirrel details between corporate America and Wall Street about what companies are up to - potentially giving some investors an unfair edge.

Federal authorities have traditionally pursued high-ranking executives and their confidants in insider-trading cases. Now, they're increasingly going after the rank and file. In September, for instance, the Securities and Exchange Commission accused a railroad supervisor and a trainman of insider trading after they noticed an "unusual number" of tours of people in "business attire" in a railyard.

The two workers and their relatives then bet in the stock market that the company would soon be taken over and made $1 million when that turned out true, according to the SEC suit.

The federal crackdown on insider trading that burst into view this week is being led by Preet Bharara, the top United States prosecutor in Manhattan. Bharara has called insider trading "rampant" and suggested it is growing.

In a strikingly revealing speech last month to the New York City Bar Association, Bharara said the detection of insider trading has "perhaps never been more difficult to attack through traditional investigative means."

The explosion of financial information, including blogs, tweets and online newsletters, makes it easier for an accused insider trader to argue that he or she was acting on information "based on some report somewhere," he said.

Bharara also spoke of the need to target insider trading by using a tool more typically deployed in racketeering cases: wiretaps.

"The question of why we use wiretaps to investigate illegal insider trading is, to my ear, like my asking a defense lawyer, why do you cross-examine the government's witnesses at trial?" he said. "Court-authorized wiretaps, so long as all the legal requirements can be met, will continue to be in our toolbox in insider trading cases."

Bharara's office was already pursuing an insider-trading case against the Galleon Group, a once-powerful hedge fund led by Raj Rajaratnam. He was charged last year with conspiring to trade insider information. He has pleaded not guilty.

Monday's raids targeted three hedge funds: Level Global Investors in New York, Diamondback Capital Management in Stamford, Conn., and an address that matched Loch Capital Management in Boston. Federal law enforcement agencies would not comment other than to confirm an investigation.

Investors can use the expert networks to glean details of what's occurring within certain industries or particular companies. Someone interested in learning more about fast-food dining in China, for example, might connect with local store managers, suppliers or experts on dining in the region.

The expert networks connect the investor and the source, getting a fee from the investor and then paying the source, who could make $400 to $500 an hour, says Sanford Bragg, CEO of the consulting firm Integrity Research Associates, which connects investors with these research firms.

Hedge funds have been paying people to dig for hard-to-find numbers on companies for years.

Tammer Kamel, president of Iluka Consulting Group Ltd. in Toronto, recalls visiting a Hong Kong fund 10 years ago that wanted to better gauge future sales by a company with factories in China. Its solution: Pay Chinese farmers near a company warehouse to count trucks leaving the site.

For a possible investment in a casino, another fund paid people to stand outside the casino and count visitors walking in, Kamel says. Then the fund multiplied that number by average losses per visitor to get a better sense of the casino's daily take.

"The managers were openly discussing technique," Kamel said. "They clearly thought it was just smart data gathering."

This week, retailer Big Lots filed a lawsuit accusing a research firm of having "wrongfully induced" stores managers to disclose "trade secrets" about the chain's inventory levels, sales and strategy. A report on those figures subsequently sent to clients "caused" Big Lots' stock to drop 6 percent, the suit alleges.

When Bharara announced arrests in the largest hedge fund insider trading case in history a year ago, targeting the head of the Galleon Group hedge fund, he said law enforcement for the first time had made extensive use of wiretaps, just like in drug cases.