Three Chinese citizens have been charged in the United States of hacking law firms to obtain confidential information on corporate mergers and acquisitions to engage in insider trading.

Iat Hong and Chin Hung of Macau and Bo Zheng of Changsha, China, were charged with conspiracy, insider trading, wire fraud and computer intrusion in an indictment filed in Manhattan federal court, Reuters reports. The three are alleged to have acquired shares in at least five publicly traded companies prior to them announcing M&A deals, managing to net over $4 million in profits through their illegally gained knowledge of the deals.

According to the indictment, the trio obtained the inside information by hacking two law firms by targeting the email accounts of partners at the firms through a phishing attack. Although not named in the indictment, the two law firms targeted are believed to be Cravath, Swaine & Moore LLP and Weil, Gotshal & Manges LLP, according to The Wall Street Journal. Five other law firms were targeted by the trio. However, it appears they weren’t able to gain access despite allegedly having tried “on more than 100,000 occasions between March and September 2015.”

Preet Bharara, U.S. Attorney for the Southern District of New York, said in a statement that the case highlights the vulnerability of highly-sensitive law firm records to cyber criminals seeking to profit from the information they hold. “This case of cyber meets securities fraud should serve as a wake-up call for law firms around the world: you are and will be targets of cyber hacking, because you have information valuable to would-be criminals,” Bharara said.

In addition to the criminal charges, the Securities and Exchange Commission has filed a separate civil lawsuit against the three accused that seeks to freeze their assets to prevent them from cashing in on their trading gains. According to USA Today, the SEC investigators used trading surveillance and analysis to identify the alleged scheme.

Of the three, only Hong has been arrested. U.S. authorities are seeking his extradition from Hong Kong, while Hung and Zheng remain at large. It’s not clear from reports whether mainland Chinese authorities have agreed to assist in their apprehension.

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