Tiffani Shea Gish from Houston, Texas, pleaded guilty to one count of using interstate communications with a threat to kidnap or injure in November, Sky News's US parter network NBC News reports.

Gish's plea came more than a year after she was arrested in connection with threatening voicemails left for US district judge Aileen Cannon.

The messages - which Gish admitted to leaving, according to court documents - said Judge Cannon was "marked for assassination" and would be shot in front of her family.

Gish has now been sentenced to 37 months in federal prison followed by three years of supervised release, the US justice department said.

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US attorney Alamdar S. Hamdani said that holding Gish accountable "sends a strong message that we have no tolerance for those [...] seeking to undermine our democratic institutions by threatening the safety of the people who help those same institutions thrive".

Separately, judges overseeing other cases involving former US president Trump have been sent threats.

In January, the judge presiding over Trump's civil fraud case was subject to a bomb threat hours before the closing arguments.

And last year, a woman was charged in connection with threats sent to US district judge Tanya Chutkan, who is overseeing Trump's federal election interference case.

Trump's legal troubles

Trump is facing multiple criminal and civil cases as he campaigns to challenge Joe Biden in this November's US election.

The classified documents case being overseen by Judge Cannon is scheduled to go to trial in May.

The case centres on classified documents discovered at Trump's Mar-a-Lago estate more than a year after he left office. He is accused of wilfully retaining national defence information.

Trump and his co-defendants, aide Walt Nauta and Mar-a-Lago maintenance supervisor Carlos De Oliveira, have denied wrongdoing and pleaded not guilty.