Cyber criminals have been targeting major law firms in what may have been an attempt to gather data for insider trading deals, according to reports.
The Wall Street Journal said that a number of US companies had had their computer systems compromised.
The FBI would not confirm whether or not an insider trading investigation was under way.
Earlier in the year, security company Flashpoint had warned some firms that hackers were targeting legal offices.
A spokeswoman for Flashpoint was not able to comment in detail on the alert sent to law firms, but told the BBC: "All the details surrounding any activity have been turned over to the authorities and they are handling the situation from there."
Law firm Cravath Swaine & Moore LLP confirmed to the BBC that it suffered a "limited breach" last summer.
"We are not aware that any of the information that may have been accessed has been used improperly," the statement said.
The rising threat of hacking has been noticed by many in the legal industry.
Recently, attempts to compromise the security of law firms' computer networks have been detected by clients of London law office CMS, which brokers cybersecurity insurance policies for a range of companies.
"We've seen examples of emails [at client law firms] that purport to come from a managing partner to a more junior lawyer directing them to make payments to an account or to send certain information to an address," Stephen Tester, a partner at CMS, told the BBC.
"They can look very much like a regular message."
Law firms had even reported breaches of their video-conferencing systems, commented Mr Tester.
"There are ways in which people can go into video-based conferencing facilities and literally listen in on meetings," he said.
He added that while the issue was a growing concern for all businesses, he felt law companies were in a special position because of the sensitivity of the data they held.
Last year, a ring of alleged insider trading hackers was apprehended after they were accused of netting more than $100m (£70m) in illegal profits.
The group was accused of stealing sensitive financial information from wire news services before it had been published.