The FBI today said it found no evidence that hackers had infiltrated the email servers of Hillary Clinton but FBI director James Comey said the department concludes that it's still "possible" sophisticated attackers could have done so undetected.

Comey in a press briefing today detailed the findings today of the FBI's investigation into Clinton's controversial use of a private email server during her tenure as US Secretary of State. The FBI recommended that no criminal charges be filed in the wake of the investigation, because there was no evidence of wrongdoing or criminal intent. But according to Comey, Clinton and her staff were “extremely careless” in their email practices.

While the FBI found no "direct evidence" of a "successful" breach of Clinton's email domain, Comey said the likely sophisticated attackers would not necessarily have been visible. "Given the nature of the system and of the actors potentially involved ... [investigators] were unlikely to see" those attacks, he said. 

The FBI also concluded that hackers did gain access to commercial email accounts of other users with whom she corresponded regularly. 

Given Clinton's practice of sending and receiving email in the "territory of sophisticated adversaries," it's "possible that hostile actors gained access to Secretary Clinton's personal email account," Comey said.

See the full text of Comey's briefing here.