MSNBC said Thursday that journalist Mark Halperin had been suspended from his role as network contributor following charges from five women that he sexually harassed them while he was an ABC News executive. The network said it found Halperin's conduct as described in a CNN story "very troubling" and that the veteran political reporter would be off the air until questions about his past were fully understood. Later Thursday, Penguin Press canceled a planned book by Halperin and John Heilemann about the 2016 election, and HBO called off a miniseries that would have been based on the book. Halperin and Heilemann had collaborated on two previous books, including Game Change, a best-seller about the 2008 race that almost single-handedly revived the campaign book genre and was the basis for an award-winning HBO adaptation. "HBO has no tolerance for sexual harassment within the company or its productions," the company said in an emailed statement. The planned book had no title or official publication date, but had been expected to be released in early 2018. Halperin has apologized for what he termed inappropriate behavior, and ABC said Thursday that no complaints were filed against him during his tenure, which ended more than a decade ago. The women, who asked to remain anonymous, said they didn't report Halperin's conduct because they feared retribution or were embarrassed. But Emily Miller, a former ABC employee who is now a reporter at One America News Network, retweeted the Halperin story on Thursday with the hashtag "#MeToo." She gave no details of her alleged incident, but tweeted that she did not report Halperin to ABC "because I thought I was the only one, and I blamed myself, and I was embarrassed and I was scared of him." CNN international correspondent Clarissa Ward, who also used to work at ABC, tweeted that Halperin's behavior was "an open secret" at the network. She saluted the women who came forward in the CNN story. "Let's be very clear — the one responsible for any sexual misconduct that may have taken place is the man who instigated it," Ward tweeted, "NOT the women who were victims of it, nor their friends and colleagues who tried to support them through it." Halperin told CNN Wednesday night that he was "deeply sorry" and was taking a "step back" from day-to-day work to deal with the situation. His statement came after CNN reported allegations that Halperin had propositioned, fondled and pushed himself against five women in the early 2000s while he was ABC News' political director. Halperin said he pursued relationships, sometimes with junior co-workers, but CNN said he denied the groping allegations. Showtime, where Halperin has co-hosted the political series The Circus, said Thursday that it had not heard of any allegations of untoward behavior. "We are aware of these reports and will continue to evaluate all options should we decide to move forward with another season of The Circus," the network said. On MSNBC's Morning Joe Thursday, co-host Mika Brzezinski informed viewers about Halperin. He has been a frequent panelist on the show. "We're going to be following this story as it develops," she said. "I'm sure we'll be talking about it again when we know more about it."