LISBON, Portugal (AP) — Police in Spain have captured a cybercrime gang made up of Ukrainians and Russians that allegedly stole more than 1 billion euros ($1.24 billion) from financial institutions worldwide in a five-year spree, authorities said Monday.

The gang's alleged mastermind, identified as a Ukrainian and named only as "Denis K.," was arrested in the coastal city of Alicante, 350 kilometers (220 miles) southeast of Madrid, according to statements issued by Spanish police and European Union law enforcement agency Europol.

Three suspected accomplices, said to be Russian and Ukrainian, were also arrested, Spanish authorities said. In Ukraine, police said that an unidentified 30-year-old man linked to the gang was cooperating with authorities.

The hackers — whose activities have long been tracked by security researchers — used malware to target more than 100 financial institutions worldwide, sometimes stealing up to 10 million euros in each heist. Almost all of Russia's banks were targeted, and about 50 of them lost money in the electronic robberies, authorities said.

The gang used well-worn techniques such as booby trapped emails to break into banks and compromise the networks controlling ATMs, effectively turning the machines into free cash dispensers.

Ross Rustici, a senior director at Boston-based digital security firm Cybereason, said the gang stood out from others because of the amount of care and planning it put into operations.

"They're unusual in how slow and methodical they are and how organized they are," Rustici said. Other groups use similar techniques in isolation, "but nobody before them had strung all those things together on such a scale," he said.

Authorities said that the gang converted its illicit gains into bitcoins and used the cryptocurrency to purchase big ticket items, including houses and vehicles, in Spain.

Rustici said it seemed likely the gang members botched their effort to launder their ill-gotten gains.

"That's usually what happens with these who are very good on the network side, they make mistakes on the money side," he said. "You can't buy a nice villa on the Mediterranean with cryptocurrency. Or at least not yet."

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Satter reported from London.