<span>Photograph: Richard Drew/AP</span>
Photograph: Richard Drew/AP

One of Fox host Tucker Carlson’s top writers has resigned after a CNN investigation found he was posting racist and sexist comments online under a pseudonym.

Under the name “CharlesXII”, Blake Neff regularly posted offensive language on an online forum called AutoAdmit.

In June, for example, Neff wrote, “Black doods staying inside playing Call of Duty is probably one of the biggest factors keeping crime down.” Neff also harassed a woman on the forum, revealing facts about her personal life and encouraging other users to post private information about her.

In an internal memo sent to staff, Fox News confirmed Neff is no longer with the broadcaster. “We want to make abundantly clear that Fox News Media strongly condemns this horrific racist, misogynistic and homophobic behavior,” read the 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.”

Carlson has called Neff a “wonderful writer”. In an interview with Dartmouth’s alumni magazine, Neff said: “Anything [Carlson is] reading off the teleprompter, the first draft was written by me.”

Carlson has not commented directly on Neff’s resignation but Fox said he will address the topic in his show on Monday. On Friday, Carlson appeared to reference Neff’s departure when he attacked what he termed “cancel culture”.

Meanwhile, others pointed out more examples of controversial journalism from Neff under his own name.

Others posted comments.

Related: Tucker Carlson calls Purple Heart recipient Tammy Duckworth a ‘coward’

Carlson, one of Donald Trump’s favourite Fox News personalities, recently repeatedly targeted the Democratic senator and Iraq war veteran Tammy Duckworth. Carlson labelled her a “coward” on Tuesday’s show after she called for a national discussion on the issue of removing US monuments and statues of historical figures who were leaders of the Confederacy or other hateful or dubious legacies.

Duckworth, a former army helicopter pilot, lost her legs after her aircraft was hit by enemy fire in Iraq in 2004.