Hackers broke into the computer networks of some big U.S. law firms, including Cravath Swaine & Moore LLP and Weil Gotshal & Manges LLP, the Wall Street Journal reported.

Federal investigators are looking to see if confidential information was stolen for insider trading, as these law firms represented Wall Street banks and big companies, the Journal said, citing people familiar with the matter.

Other law firms were also targeted, but the probe has not amounted to any clear information on what details have been stolen, the newspaper reported.

The Federal Bureau of investigation and the Manhattan U.S. attorney's office are investigating the matter, WSJ said.

Hackers have threatened more such attacks in postings on the Internet, the Journal said.

Cravath Swaine & Moore and Weil Gotshal & Manges were not immediately available for comment outside regular business hours.