(CNN) -- Mel Gibson frequently spews "looney, rancid" anti-Semitism, has talked about killing his former girlfriend, and is prone to hate-filled diatribes slamming everyone from John Lennon to Walter Cronkite, according to a screenwriter who has been working with him.
Joe Eszterhas, who wrote a screenplay about the Jewish hero Judah Maccabee for Gibson, recounts numerous alleged incidents in detail in a nine-page letter to Gibson published by the website thewrap.com.
In a letter replying to Eszterhas, Gibson denies the allegations, saying most of the claims are fabricated.
Gibson's letter says Eszterhas "only had a problem with me after Warner Brothers rejected your (Eszterhas') script."
Gibson, in his five-paragraph response, says he won't respond "line by line," and that the decision not to proceed with Eszterhas "was based on the quality of your script, not on any other factor."
A spokesman for Gibson, Alan Nierob, gave CNN a copy of the letter and said Gibson will have no further comment at this time.
Gibson's bitter child custody battle with former girlfriend Oksana Grigorieva ended last year with a court settlement, but not before the actor entered a "no contest" plea to a misdemeanor battery charge relating to a 2010 incident involving Grigorieva.
The plea deal put Gibson on unsupervised probation for three years.
Grigorieva's spokesman told CNN Thursday that if the Eszterhas "is accurate and credible, the allegations are extremely serious and must be investigated immediately by the authorities."
"But it will be up to the authorities and Ms. Grigorieva's attorneys to react to this revelation," Steve Jaffe said. He confirmed she has "been in touch" with her lawyers about the matter.
Warner Bros. has put the controversial Maccabees project on hold, the company said. "We are analyzing what to do with the project," said spokesman Paul McGuire. Like CNN, Warner Bros. is part of Time Warner.
Eszterhas writes in his letter, "I've come to the conclusion that the reason you won't make 'The Maccabees' is the ugliest possible one. You hate Jews." He recounts Gibson repeatedly using derogatory epithets for Jewish people.
Allegations of anti-Semitism are nothing new for Gibson. Concerns that arose among some Jewish groups over his handling of the story of Jesus in "The Passion of the Christ" in 2004 were replaced by widespread condemnations two years later when Gibson was arrested on a drunk driving charge. According to a police report, he asked the arresting officer if he was Jewish and said, "F***ing Jews. The Jews are responsible for all the wars in the world."
Gibson later apologized without acknowledging specific remarks.
Eszterhas is a veteran of the industry, having penned such titles as "Basic Instinct" and "Showgirls."
In his letter, he writes that he hoped Gibson viewed the Maccabees project "as a kind of penance/apologia" -- a claim Gibson denies in his response.
Eszterhas says that soon after he began working with Gibson on it, he became "increasingly worried that I'd made a grave mistake by hooking up with you."
It was not immediately clear how thewrap.com obtained Eszterhas' letter.
In discussing Jewish people, Gibson regularly used the terms "Hebes," "oven-dodgers," and "Jew-boys," Eszterhas alleges. "You said most 'gatekeepers' of American companies were 'Hebes' who 'controlled their bosses.'"
"You said the Holocaust was 'mostly a lot of horsesh*t,'" the letter says, adding that Gibson made various false accusations, including that the Torah refers to sacrificing Christian babies.
Gibson called Pope John Paul II "the anti-Christ" and "the devil," the screenwriter alleges.
"You kept raging about your ex-girlfriend, Oksana Grigorieva," mother of their young daughter Luci, the letter alleges, saying Gibson referred to her with sexist epithets. "You acted out for me the scene where you hit her. But you said you'd 'just slapped her a little bit.'"
Eszterhas claims in the letter Gibson explicitly said, "'I'm going to kill her! I'm going to have her killed!'"
Addressing Gibson, he says in the letter, "You said you'd become friends with two FBI agents (or former FBI agents) and they were going to help you to kill her."
A spokesman for Grigorieva did not immediately respond to a call from CNN Thursday.
Eszterhas recounts times that he, his wife, and his 15-year-old son felt endangered in Gibson's presence.
The teen taped one of Gibson's outbursts on his iPod, Eszterhas says.
Gibson once told the 15-year-old that he wanted to perform a sex act on Grigorieva and "stab her to death while I'm doing it," Eszterhas alleges.
"How can you share a loop from the pornographic snuff film which obviously plays in your head ... with a child?" the letter asks.
The bitter battle between Gibson and Grigorieva reached a financial and custody settlement last year. Gibson pleaded no contest to a charge of misdemeanor domestic battery.
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