A sophisticated breach of U.S. telecommunications systems has extended to the presidential campaigns, raising questions about the group behind the attack and the extent of its efforts at collecting intelligence.

It was unclear what data was taken in the attack. The far-reaching operation has been linked to the Chinese government and attributed to a group experts call Salt Typhoon. Investigators believe hackers took aim at a host of well-connected Americans, including the presidential candidates — reflecting the scope and potential severity of the hack.

Here’s what to know.