As first reported by the New York Times, an international group of hackers has allegedly stolen as much as $1 billion from banks around the world, one of the largest bank heists ever.

The hackers used elaborate phishing schemes to impersonate bank employees, delivering malware through attachments which then lay dormant for months gathering key information. Thus far no banks have acknowledged the theft.

At the White House Summit on Cybersecurity last week, President Obama called for a new law requiring public disclosure  in cases of compromised personal or financial data.

Correction: A previous version of this story misstated how much hackers were able to take from the banks. It’s unclear exactly how much the thieves made off with, but the cybersecurity company reporting the breach said they could have taken up to $1 billion.