Two people briefed on the probe said that because just about any SolarWinds customer that used the product got the manipulated software, the number of victims could eventually reach into the thousands. The hackers appear to have concentrated on the most attractive and sensitive targets first, so that the harm suffered by various victims may vary widely, according to the people, who asked not to be identified because the information isn't public. FireEye told clients on Sunday that it was aware of at least 25 entities hit by the attack, according to people briefed by the company. The quickly broadening investigation broke into public view on December 8 when FireEye announced that it had been breached in a highly sophisticated attack that it attributed to hackers backed by US adversaries. FireEye uncovered the hack in the course of investigating the breach.