The FBI obtained a warrant to begin new probe into Clinton emails
The FBI's director has been warned he could have broken US law by announcing an investigation into emails potentially linked to Hillary Clinton.
A senior Democrat is accusing FBI director James Comey of illegally influencing the upcoming presidential election, as they begin reviewing emails from a Clinton aide.
In a scathing letter, minority Senate leader Harry Reid wrote: “Your actions in recent months have demonstrated a disturbing double standard for the treatment of sensitive information, with what appears to be a clear intent to aid one political party over another."
The Senator from Nevada also stated that Comey has "explosive information about close ties and coordination between Donald Trump, his top advisors, and the Russian government," which has not been released yet.
According to The Wall Street Journal, the probe is looking into around 650,000 messages found on the laptop of former congressman Anthony Weiner - who is married to one of Mrs Clinton's top aides.
Its article reported that a review lasting "weeks" would examine the emails' content, whether they were duplicates of messages already seen by the FBI, and whether they contained classified information.
A poll shows Mrs Clinton's lead over Republican candidate Donald Trump has narrowed in light of the probe.
The findings from Real Clear Politics show the democrat has 47.6% support among voters - 4.3% than her Republican rival. Clinton led by 5.2% on Friday when the new investigation was announced.
Speaking at a rally in New Mexico, Donald Trump has again used the revelations to criticise his rival.
"How do you have that many emails? What do you do, just sit down all day and keep typing?" he said. "No wonder nothing gets done in our country."
Hillary Clinton's campaign manager Robby Mook's told Fox News the timing is disturbing.
"This issue was vetted through for months. We believe that voters have already made up their minds on this issue, so we find it very strange that he [Comey] is putting this letter out there," he said.
Mrs Clinton herself has called on Mr Comey to put out the "full and complete facts" about the fresh FBI review.
The Democratic presidential nominee stepped up her attack on Saturday night, describing Mr Comey's decision as "deeply troubling".
At a rally in Florida on Sunday, she told supporters: "When you're knocked down what matters is whether you get up again. I'm not stopping now."