US intelligence agencies are reported to be examining suspected fake documents apparently aimed at discrediting Democratic nominee Hillary Clinton's campaign.
The inquiry is understood to be part of a broader investigation into what officials believe is an attempt by Russia to disrupt the election.
US Senator Tom Carper, a Democrat on the Senate Homeland Security Committee, referred one of the documents to the Federal Bureau of Investigation (FBI) for investigation on the grounds that his name and stationery had been forged to appear authentic, some of the sources who had knowledge of that discussion said.
In the letter identified as fake, Mr Carper was quoted as writing to Mrs Clinton, "We will not let you lose this election," a person who saw the document told Reuters.
The letter, which was described to Reuters, was one of several documents presented to the FBI and the US Department of Justice for review in recent weeks, the sources said.
A spokesperson for Mr Carper declined to comment.
As part of an investigation into suspected Russian hacking, FBI investigators have also asked Democratic Party officials to provide copies of other suspected faked documents that have been circulating along with emails and other legitimate documents taken in the hack, people involved in those conversations said.
An FBI spokesperson confirmed the agency was "in receipt of a complaint about an alleged fake letter" related to the election but declined further comment. Others with knowledge of the matter said the FBI was also examining other documents that recently surfaced.
US intelligence officials have warned privately that a campaign they believe is backed by the Russian government to undermine the credibility of the presidential election could move beyond the hacking of Democratic Party email systems.
That could include posting fictional evidence of voter fraud or other disinformation in the lead-up to voting on 8 November, US officials have said. Russian officials denied any such effort.
In addition to the Carper letter, the FBI has also reviewed a seven-page electronic document that carries the logos of Democratic pollster Joel Benenson's firm, the Benenson Strategy Group, and the Clinton Foundation, a person with knowledge of the matter said.
The document, identified as a fake by the Clinton campaign, claimed poll ratings had plunged for Mrs Clinton and called for "severe strategy changes for November" that could include "staged civil unrest" and "radiological attack" with dirty bombs to disrupt the vote.
Like the Carper letter, it was not immediately clear where the fraudulent document had originated or how it had begun to circulate.
On 20 October, former Trump aide and Republican operative Roger Stone linked a copy of the document on Twitter with the tag, "If this is real: OMG!!"
Mr Benenson's firm had no immediate comment. Craig Minassian, a spokesperson for the Clinton Foundation, said the document was "fake". He said he did not know if the FBI had examined it.
Mr Stone did not respond to emails requesting comment.
Clinton campaign spokesperson Glen Caplin said the document was a fake and part of a "desperate stunt" to capitalise on the leak of Democratic emails by WikiLeaks.
The developments highlight the unusually prominent role US law enforcement and intelligence agencies have played in a contentious election and an ongoing debate about how public they can or should be about their inquiries.
FBI Director James Comey, a Republican appointed by US President Barack Obama, touched off an outcry from Democrats last week when he alerted Congress that agents had found other emails that could be linked to an inquiry into Mrs Clinton's use of a private email server when she was Secretary of State, effectively re-opening an investigation he had closed in July.