The Internal Revenue Service data breach continues to develop, as the agency said Friday that approximately 724,000 taxpayers' information was probably stolen during last year's hack.

The IRS was hit by hackers in May of last year, with the IRS stating several months later that data of approximately 334,000 victims had been accessed. The latest revelation more than doubles the number of people whose personal details could have been stolen.

The hackers had used an IRS application called "Get Transcript" to access the data. Experts believe that social security numbers, birth dates and other relevant data were stolen in order to facilitate the impersonation of real tax payers to collect tax refunds.

"TIGTA investigators identified suspicious email addresses that made multiple attempts to access accounts. It is possible that some of those identified may be family members, tax return preparers, or financial institutions using a single email address to attempt to access more than one account," the IRS said in a statement on Friday, according to Wired.

"The IRS is committed to protecting taxpayers on multiple fronts against tax-related identity theft, and these mailings are part of that effort," IRS Commissioner John Koskinen said, according to USA Today.

"We appreciate the work of the Treasury Inspector General for Tax Administration to identify these additional taxpayers whose accounts may have been accessed. We are moving quickly to help these taxpayers," Koskinen added.

 The "Get Transcript" feature has been disabled by the IRS since the attack last year.