Suspected Chinese government-backed hackers have breached computer systems of U.S. law firm Williams & Connolly, which has represented some of America’s most powerful politicians, as part of a larger spying campaign against multiple law firms, according to a letter the firm sent clients and a source familiar with the hack.

The cyber intrusions have hit the email accounts of select attorneys at these law firms, as Beijing continues a broader effort to gather intelligence to support its multi-front competition with the U.S. on issues ranging from national security to trade, multiple sources have told CNN.

The hackers in this case used a previously unknown software flaw, coveted by spies because it allows for stealth, to access Williams & Connolly’s computer network, said the letter sent to

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