An IRS cyber hack perpetrated in 2015 could have compromised personal data for over 700,000 taxpayer accounts which is more than twice the previous estimate, the tax agency revealed Friday.

There is now greater chances that the cyber thieves could impersonate thousands of taxpayers since the illegally obtained information include specific data such as birth dates and Social Security numbers. Thieves could use this information to collect tax refunds, file false federal tax returns, and many more tax related illegal activities.

Previously, the tax agency reported in May 2015 that it found 114,000 US taxpayer data were illegally accessed using the "Get Transcript" since January 2014.

The "Get Transcript" application enables individuals to view information from the tax returns of the previous years. After the cyber-attack was uncovered in May, the IRS website app was immediately discontinued.

This attack of the still unidentified hackers gave a 'black eye' to the IRS. It took information on taxpayers taken elsewhere and used it to answer personal identity-verification questions correctly which were asked by the "Get Transcript" software in the IRS website.

Originally, the app allowed legitimate taxpayers to access information about the tax filings they did in the past. Now, the IRS has completely disabled the program.

Initially, the tax agency said that about 100,000 taxpayer accounts were compromised, but in August, it raised the total to as many as 334,000. The estimate given by the IRS on Friday has an additional 390,000, making a total of more than 700,000 illegally accessed taxpayers' personal accounts.

The hackers attempted to access an additional 500,000 taxpayer accounts but they were unsuccessful said the IRS. John Koskinen, the IRS Commissioner stated that the agency is planning to mail notifications and assistance offers to taxpayers whose accounts appeared to have been illegally accessed.

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