The 41-year-old Labour MP for Batley and Spen died yesterday after she was stabbed and shot outside a library in Birstall.
Her death comes three months after a man was cautioned for sending her ‘malicious communications’.
Police have since confirmed the abuse messages were not the same man who was arrested on Thursday following the attack.
Witnesses said her attacker kicked and stabbed her and then shot her several times, the final shot aimed at her head.
The alleged gunman has been named locally as Tommy Mair, 52.
He reportedly shouted ‘Britain First’ during the incident – though the group have denied any involvement.
He was arrested near the scene soon after the attack.
Mrs Cox had previously complained to police about receiving malicious communications.
A Metropolitan Police spokeswoman said: ‘Officers received an allegation of malicious communications from Jo Cox MP, and in March 2016 arrested a man in connection with the investigation.
‘The man subsequently accepted a police caution. The man who accepted the police caution is not the man in custody in West Yorkshire.’
The attack on Mrs Cox comes after female MPs said security was a ‘key concern’ when they were surveyed last August.