The FBI raided the offices of three hedge funds Monday, beginning its investigation into insider trading, The Wall Street Journal reports, citing people familiar with the matter. The FBI is planning to raid additional hedge funds, they said.

The FBI raided the offices of Diamondback Capital Management and Level Global Investors, both located in Connecticut and run by former managers of SAC Capital Advisors. The third hedge fund raided, Loch Capital Management, is in Boston.

“The FBI is executing court-authorized search warrants in an ongoing investigation,” FBI spokesman Richard Kolko told The Journal.

The biggest hedge fund of the three is Diamondback, with $5 billion in assets under management. Level Global has $4 billion, and Loch $750 million.

Reports came out over the weekend that the FBI is looking into whether “expert network” consultants passed insider trading information to hedge funds and mutual funds. The FBI reportedly believes this investigation, three years in the making, will surpass any insider trading case to date.