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The criminal case against former Trump adviser George Papadopoulos is just a "small part" of special counsel Robert Mueller's investigation into Russian meddling in the 2016 presidential election, prosecutors said earlier this month.

The intriguing comment came during Papadopoulos' secret plea hearing on Oct. 5 during a discussion of the need for records to be kept under wraps until the probe is completed.

"The criminal justice interest being vindicated here is there's a large scale ongoing investigation of which this case is a small part," prosecutor Aaron Zelinsky told the judge, according to a transcript of the proceeding.

Papadopoulos was arrested July 27 when he landed in Washington on a flight from Munich, Germany. Prosecutors asked that his arrest be sealed because any disclosure could "undermine his ability to serve as a proactive cooperator" or prompt other targets to "destroy or conceal incriminating evidence," according to newly unsealed documents.

Papadopoulos' plea and cooperation agreement with Mueller's office weren't announced until Monday, shortly after indictments were unsealed against former Trump campaign chair Paul Manafort and his associate, Rick Gates.

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Related: From Top Trump Campaign Aides to Accused Partners in Crime

While the tax fraud and money laundering charges against Manafort and Gates stem from their business dealings, the Papadopoulos case is centered on the campaign and his efforts to set up a meeting between Trump and the Russians.

The 30-year-old energy expert admits he lied to FBI agents about the timing, extent and nature of his contacts with foreign nationals who had ties to the Kremlin — claiming he had met them before he joined the campaign in March 2016.

One of his contacts, a London-based professor who introduced him to a well-connected Russian, allegedly told Papadopoulos that Moscow had "dirt" on Hillary Clinton — including "thousands of emails" — well before the hacking of Democratic Party figures was publicly revealed.

Initially charged with making false statements and deleting records from his social media, Papadopoulos pleaded guilty only to lying in a sign of the government's willingness to cut him a break for his continued cooperation with the larger investigation.

Prosecutors said during the plea hearing that they shared "substantial information" with Papadopoulos that "provided a road map of sorts" to the ongoing investigation in an attempt to "jog his memory" as he cooperated.

It's not clear what information Papadopoulos may have provided that is not in court papers, but Trump and his allies reacted to the surprise news of his plea deal by portraying him as a campaign peon.

"Few people knew the young, low level volunteer named George, who has already proven to be a liar," the president tweeted on Tuesday.

Former campaign manager Corey Lewandowski said Papadopoulos "was not a person who was involved with the day-to-day operations of the campaign," though he could not recall if he received emails from him about Russia.

Court documents say campaign officials were aware of Papadopoulos' efforts; he discussed his connections at a March 31, 2016 "national security meeting" attended by then-candidate Trump and future Attorney General Jeff Sessions.

In April and in June, he emailed a high-ranking campaign official about Russia's interest in meeting Trump, according to court documents. The official — who was identified as Manafort by two sources familiar with the case — forwarded one of the emails with a note that said: "We need someone to communicate that DT is not doing these trips. It should be someone low level in the campaign so as not to send any signal."