EXCLUSIVE: The Romanian hacker who says he easily breached Hillary Clinton’s personal email server also claimed, in a series of interviews with Fox News, that he spoke with the FBI at length on the plane when extradited from Romania to Virginia last month.

"They came after me, a guy from the FBI, from the State Department," 44-year-old Marcel Lehel Lazar, who goes by the moniker "Guccifer," told Fox News during a jailhouse phone interview. He said the conversation was "80 minutes ... recorded," and he took his own notes.

A government source confirmed that the hacker had a lot to say on the plane but provided no other details. Lazar was flown to the U.S. to face separate cyber-crime charges.

In addition to the apparent conversation with the FBI on the plane, Fox News has learned a meeting was expected as early as this week at the Alexandria, Va., detention center where he’s being held involving Guccifer, the FBI, the U.S. attorney and the defendant’s court-appointed lawyer.

These officials have not commented on his clams or detention.

An intelligence source close to the investigation, speaking with Fox News last month, questioned the timing of Lazar’s extradition to the U.S., coming amid the Clinton email probe. As for what was discussed on that plane, Lazar said he told a State Department representative on the plane about "hot" data, some of which was hidden in Google drives, and other data that was too sensitive and deleted. The hacker, who offered no proof for his claims, said cryptically that he could not say more.

"I can't tell [you] now. I can't tell because I want to talk to the FBI. It is a matter of national security. Yeah," he said. Pressed by Fox News, Lazar seemed to indicate the data was not connected to the ongoing FBI criminal probe of Clinton's server.

Fox News recently met with Lazar in the secure visitor center in Alexandria, then followed up with a series of phone calls which he gave permission to be recorded. Separated by reinforced glass, Lazar was polite and methodical as he explained how he allegedly accessed the Clinton server in early 2013, by using her longtime confidant Sidney Blumenthal's AOL account as a stepping stone.

Fox News was first to report the hacker's claims of accessing the Clinton server, which he said “was easy.”

Lazar said he got into the Blumenthal account by correctly guessing his security question, after doing extensive research on the web. He said his hacking always followed a "four step process": identify the target, do extensive web research on the target, access the target's account to harvest data, and send it out to the media.

Lazar said he was puzzled by the American media. He said he sent the Blumenthal emails, which is how the Clintonemail.com account first came to light, to many large news organizations in 2013, and it was The Smoking Gun that picked it up. Lazar said he started his "Guccifer archive," releasing materials in October and November 2012, and it ended "like August 2013."

Three cybersecurity experts said they found Lazar's explanation for accessing the Clinton server plausible but had questions.

Cybersecurity expert Morgan Wright explained how the FBI could marry up available evidence, including forensics or the configuration of the server and its folders, to assess his claims. "So we're going to map these things together, and if those things match up together, they're going to say ‘yes, this was compromised,’ then it means it was open to other people to compromise as well," he said.

Since Fox News reported on Guccifer’s claims Wednesday, anonymous sources have reported that a review of the Clinton hard drives does not appear to indicate a breach. However, Wright and other experts warned that Clinton IT specialist Bryan Pagliano was the server's administrator and not principally a cybersecurity specialist – and may not have installed an adequate detection system for a Cabinet secretary’s email.

“If you have a bank and you have one video camera when you need 20, then you missed it,” Wright said. “If they weren't capturing all the activity, their security logs may say they didn’t see anything."

Asked about Lazar’s claims at Thursday’s press briefing, State Department spokesman Mark Toner also said he’s not aware of such an incident.

“We don’t have any reason to believe that it might be true,” he said.

At the same time, Toner repeatedly stressed he did not want to comment on the security of the server, citing ongoing investigations. Asked if he’s in a position to even know whether Lazar’s claims are true, Toner again said he did not want to comment. The Clinton campaign has rejected Lazar’s claims, calling them “baseless” and emphasizing he is a convicted hacker.

Other cyber specialists like Bob Gourley with Cognitio warned there will “always be uncertainty and ambiguity” with hackers like Guccifer. But he said: “One thing I would say with certainty however -- if this computer were in a well-managed facility, where everything was being monitored and watched, we would have more information and ground truth."