Computer crime experts at the National Hi-Tech Crime Unit (NHTCU) have been called in to help investigate an attempted DoS attack on the Greater Manchester Police (GMP).
GMP chief constable Michael Todd was bombarded with 2,000 emails per hour at the beginning of May in what is believed to have been an attempt to crash the force's computer systems.
The emails were spoofed to show US President George Bush as the sender and warned Todd the attacker knew where the police chief and his family lived.
GMP's systems withstood the email attack and the force immediately launched an investigation.
But after struggling to get any concrete leads, the force has now called in cybercrime experts from the NHTCU in the hunt for the attacker.
A GMP spokesman said: "Enquiries are still ongoing in relation to the cyberattack against GMP's email system earlier this year."
Cambridgeshire Police were subject to a similar attack almost two years ago when thousands of spam emails told recipients their credit cards were about to be charged for an iPod they had just bought unless they phoned a customer service number.
The customer service number turned out to be the switchboard at Cambridgeshire police, which was deluged with thousands of calls from people who had received the hoax email, although the culprit was eventually tracked down and arrested.