Washington
According to the probe, there were many posts under the pseudonym including derogatory comments African-Americans, Asian-Americans and women.
He also maintained a thread harassing a woman and posted information about her personal life.
The posts came to light after CNN received a tip-off that CharlesXII was in fact Neff.
They were able to identify him by matching up content in his messages with publicly available information about him, the BBC reported.
Responding to the development, Fox News said in an internal memo: "Neff's abhorrent conduct on this forum was never divulged to the show or the network until Friday, at which point we swiftly accepted his resignation.
"Make no mistake, actions such as his cannot and will not be tolerated at any time in any part of our work force."
The network said Carlson would address the topic in his show on Monday.
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