Emails obtained by Fox News reveal that former IRS official Lois Lerner warned colleagues to be cautious about what they were saying in emails.

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Rep. Darrell Issa, R-Calif., referred to that discussion during a House oversight committee hearing on Wednesday, as he once again pressed IRS Commissioner John Koskinen on his concerns about ex-official Lois Lerner's missing emails.

He specifically voiced concern about an internal IRS chat system called the Office Communications Server, claiming Lerner wanted to make sure the messages weren't tracked.

This system was the subject of the emails between Lerner and other staffers in April 2013.

On April 9, 2013, Lerner, the director of Exempt Organization at the IRS, sent an email to IT employee Maria Hooke, asking about the OCS and whether Congress could access those messages.

"I was cautioning folks about email and how we have had several occasions where Congress has asked for emails and there has been an electronic search for responsive emails -- so we need to be cautious about what we say in emails," Lerner wrote.

She forwarded a question about whether the chat system conversations were also searchable.

Hooke responded that the messages "are not set to automatically save" but anyone could "copy and save the contents" to an email or file.

She recommended that the agency treat these conversations "as if it could/is being saved [somewhere], as it is possible for either party of the conversation to retain the information and have it turn up as part of the electronic search."

Lerner replied: "Perfect."

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Rep. Issa discussed the latest developments in the IRS scandal this morning with Brian Kilmeade, saying House Republicans have now found proof of Lerner trying to cover her tracks.

"She was trying to make sure that on April 9th of 2013 that her people weren't being tracked. ... Why? She didn't want an audit trail of what they were doing. And what they were doing were targeting conservatives for their views. No question at all," said Issa.

Issa also highlighted the fact that in the emails, Lerner "delighted" in the fact that instant messages being used by the IRS would not be recoverable later on.