Two police forces are looking into "racist and threatening" emails and letters sent to the disgraced Peterborough MP, Fiona Onasanya.
Onasanya was released from prison on Tuesday - less than four weeks after she was sentenced for lying to police over a speeding ticket.
Cambridgeshire Police confirmed that it and South Yorkshire Police were looking into "malicious communications".
A spokesman for Onasanya described the threats as "despicable".
The 35-year-old solicitor was expelled by the Labour Party in December, but remains an independent MP for Peterborough.
The nature of the threats has not been revealed, although the BBC understands they were made around the time Onasanya was jailed last month.
Onasanya's communications officer, Matthew Turner, said: "Many of these emails and letters have a disgusting racial element to them.
"This despicable and threatening behaviour is unacceptable and we welcome an investigation."
Onasanya denied being behind the wheel when her Nissan Micra was spotted being driven at 41mph in a 30mph zone in Thorney, Cambridgeshire, in July.
She was found guilty of perverting the course of justice at the Old Bailey, and was jailed for three months on 29 January.
Her release comes a day after the attorney general's office rejected a complaint which said the sentence given to her was unduly lenient.
Onasanya will now be entitled to take part in key Brexit votes in the Commons while wearing an electronic tag, according to the Daily Mail.
However, constituents in Peterborough have criticised her early release and called for the MP to resign.
"We haven't had a representative through all the Brexit issues," one woman told BBC Radio Cambridgeshire, "and she should do the honest thing."