Having that information enabled the hackers to clear a multi-step authentication process. This included their ability to correctly answer personal verification questions that are supposed to be known only by the taxpayer. The IRS says there were more than 200,000 attempts to access data and began in February and lasted through mid-May with half of these successful.

The accounts do not involve the main computer system that handles tax filing.

Get Transcripts was shut down after the IRS detected the illegal attempts this month and it will remain offline until modifications are made the IRS strengthens security. The incidents are under review by the Treasury Inspector General for Tax Administration as IRS' Criminal Investigation unit.

The IRS says those whose accounts were not penetrated will be notified because the malefactors have personal data of the other 100,000 taxpayers. The IRS is offering free credit monitoring to those whose accounts were compromised to make sure the information is not used elsewhere. Letters include details about credit monitoring and other steps.

The IRS will match the accounts hacked against those that filed taxes this year as it's possible stolen information might be used to file returns next year.