One of the biggest networks of spam-sending computers in the world has gone quiet, puzzling experts, internet security firms have said.
For years the Necurs botnet has distributed junk mail and malware for many different groups of cyber-thieves.
But the amount of malicious traffic emerging from Necurs has now dwindled to almost nothing.
It is not clear what has caused the slowdown and whether traffic will return to previously high levels.
One of the first signs of the disruption was seen earlier this month when email messages spreading the Dridex banking trojan and Locky ransomware caught by security firms dried up.
Typically, millions of messages bearing these malicious programs are sent out every week, Proofpoint said in a blogpost, external.
However, the flood of messages "essentially stopped" last week, it said. Investigations revealed that these messages typically travelled via the Necurs botnet which was found to have gone largely offline.