Those new emails were discovered during an FBI investigation in which the devices of former Congressman Anthony Weiner and Clinton aide Huma Abedin were seized, NBC News confirmed.
While examining Weiner's laptop, investigators discovered Abedin also used the laptop, which contained some emails between Abedin and Clinton.
"In previous congressional testimony, I referred to the fact that the Federal Bureau of Investigation (FBI) had completed its investigation of former Secretary Clinton's personal email server. Due to recent developments, I am writing to supplement my previous testimony," Comey wrote.
"In connection with an unrelated case, the FBI has learned of the existence of emails that appear to be pertinent to the investigation. I am writing to inform you that the investigative team briefed me on this yesterday, and I agreed that the FBI should take appropriate investigative steps designed to allow investigators to review these emails to determine whether they contain classified information, as well as to assess their importance to our investigation," he added.