Russian police have reportedly arrested a man on suspicion of masterminding two infamous hacking tools.
He is suspected of being the man behind the alias Paunch - the nickname used by the creator of the Blackhole and Cool exploit kits, sold to cybercriminals to infect web users with malware.
The Russian authorities have not confirmed the details.
But security firms said they had already detected a decline in the programs' use.
A spokesman for the law enforcement agency Europol told the BBC: "Europol and the European Cybercrime Centre has been informed that a high-level suspected cyber criminal has been arrested.
"We can only refer you to the Russian authorities, they are the ones who should speak about this topic."
The Russian police's press office said it had nothing to add at this time.
However, Alexander Gostev, chief security expert at the Moscow-based internet protection provider Kaspersky Lab, said the arrest had been confirmed to him by "anonymous sources".
The Blackhole kit, released in 2010, dominated the crimeware market throughout 2012 and the start of 2013, according to Fraser Howard, a researcher at the anti-virus company Sophos.
He said the code had been sold for an annual licence of $1,500 (£940) or could be rented from its creator for $200 (£125) for one week's use, among other price plans.
The software targeted a range of vulnerabilities in the Java programming language, Adobe's Flash media player, Windows software and PDF files.
It had two ways of doing this:
adding malicious code to hundreds of thousands of legitimate websites, which then copied malware to visitors computers
creating links in spam messages to specially created sites that infected PCs
Blackhole email Sophos said that Blackhole was used to send links that directed users to sites that downloaded malware
Among the malware downloaded was:
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