Even as Hillary Clinton tries to put questions about her private email server behind her, the FBI has stepped up inquiries into the security of the former secretary of state's home-made email system and how aides communicated over email, POLITICO has learned.

The FBI's recent moves suggest that its inquiry could have evolved from the preliminary fact-finding stage that the agency launches when it receives a credible referral, according to former FBI and Justice Department officials interviewed by POLITICO.

"This sounds to me like it's more than a preliminary inquiry; it sounds like a full-blown investigation," said Tom Fuentes, former assistant director of the FBI. "When you have this amount of resources going into it …. I think it's at the investigative level."

The FBI declined to respond to questions about the scope of its ongoing work.