The Federal Bureau of Investigation is ramping up inquiries into the security of former Secretary of State Hillary Clinton's private home-brew email server, indicating that the bureau is moving away from a preliminary fact-finding mission and into a full-fledged investigation, reports Politico.

The FBI is conducting a greater number of probes and interviews to determine whether Clinton or her aides jeopardized national security secrets with their unsecured email use, and if so, who should be held responsible for exposing classified data, former FBI and Justice Department officials said, reported The International Business Times.

The website said it learned that in early October, the FBI requested documents from a company involved in handling Clinton's server after she left office. A high-ranking State Department official also told Politico that they were interviewed by the FBI about the contents of top Clinton aides' emails.

The questions focused on whether anyone at the State Department was worried that Clinton's email practices could put classified information at risk, according to the official, who spoke on the condition of anonymity.

Politico notes that the State Department official interviewed by the FBI had little to do with Clinton's server arrangement or any approval process that granted her permission to use the personal server for official government business, which suggests that the FBI is expanding its investigation and looking into the contents of messages shared by staff.

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

While the investigation may have moved past the preliminary stage, the FBI is not required to announce such a decision.

Ron Hosko, former assistant director of the FBI's Criminal Investigation Division, told Politico that the Justice Department may be worried about issuing formal legal notices "because they know it will get out, and then you're talking about a grand jury investigation."

In formal investigations, companies will often comply only when issued a subpoena, court order or other legal notice, or else risk their corporate reputation, according to Politico.

"I am sure there is hand-wringing and gnashing of teeth across the street at the Hoover Building because you're going to have people saying 'I don't want to produce X documents. Give me a piece of paper that covers me.' And that's where push is going to come to shove," Hosko added.

The Justice Department launched its investigation after receiving a referral from the Intelligence Community Inspector General, who said he had found two emails on Clinton's server containing "top secret" material, reports The Daily Caller. Hundreds of other emails have been retroactively classified, but Clinton and the State Department insist that none of the material was marked classified at the time of origin.