Guccifer, who targeted high-profile US politicians, pleaded guilty in May on charges of aggravated identify theft and unauthorized access of a computer.
Among targeted politicians are former Secretary of State Colin Powell and the Bush family.
Marcel Lazar, 44, was arrested on hacking charges in Romania in 2014.
He was extradited to the US to face charges in March 2016.
Guccifer inadvertently uncovered Hillary Clinton’s private email account in 2013 after hacking the account of her former political adviser, Sidney Blumenthal.
Two years later, it was revealed that Hillary Clinton exclusively used her personal account, which was connected to a private server at her New York residence, while serving as secretary of state.
Hillary Clinton’s use of private email while at the State Department became the subject of an FBI investigation and has dogged her on the campaign trail.
Earlier this year Guccifer claimed he had hacked into Hillary Clinton’s private server, but the Clinton campaign has denied there was any such breach.
The FBI also has said there is no evidence to prove Marcel Lazar had hacked her.
Guccifer, a former taxi driver in Romania, is believed to have hacked into about 100 US email accounts between 2012 and 2014.
In June 2016, a hacker who goes by the alias “Guccifer 2.0” claimed data breaches on the Democratic National Committee and the Democratic Congressional Campaign Committee.
US intelligence officials have said they believe Guccifer 2.0 is linked to Russian intelligence services, suggesting that the hack was state-sponsored.
There is no known connection between Marcel Lazar and Guccifer 2.0.
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