A recently observed global decline in spam e-mail could prove to be short lived, researchers say.
A network of infected computers used to produce spam has sprung back to life after a lull of several weeks.
The Rustock botnet - as it is known - mysteriously stopped sending spam e-mails in early December, resulting in a massive decline in spam.
But according to security firm NetWitness, it restarted activity early on 10 January.
Overall spam levels are still below their levels in August 2010, when Rustock began to wind down its activity.
The reason for the lull was not immediately apparent, Alex Cox of NetWitness told BBC News.
Those controlling Rustock do not appear to have made major strategic changes to their spamming campaigns, he added.