SAN FRANCISCO (CBS SF) — The IRS admitted Monday that a massive data breach was much larger than first thought.

The agency says hackers may have accessed as many as 334,000 taxpayer accounts, more than double the number of potential victims reported in May.

It’s believed the hackers are using the Internal Revenue Service’s website to get information on victims, in order to obtain fraudulent tax returns.

The IRS will begin notifying newly discovered victims in the next few days.