21st Century Fox  (FOXA) has enlisted the law firm Paul, Weiss, Rifkind, Wharton & Garrison to investigate sexual harassment claims against Bill O'Reilly, the face of its most popular show, "The O'Reilly Factor." 

The move follows a New York Times report published last week which revealed that he and Fox News had paid $13 million to settle claims filed by five women who complained of sexual harassment and other behavior. The Fox investigation was put in motion by a complaint filed by Wendy Walsh, once a regular guest on the "O'Reilly Factor," on the network's anonymous hotline last week. 

Shares of Fox were up 0.2% to $31.12 on Monday morning, but have fallen about 3% over the last five days, compared to the S&P 500's flat performance over the same time. 

"21st Century Fox investigates all complaints and we have asked the law firm Paul Weiss to continue assisting the company in these serious matters," a Fox spokesperson said in a statement.

Paul Weiss also conducted the internal investigation into Roger Ailes, the longtime head of the conservative news network, after former Fox News anchor Gretchen Carlson filed a complaint alleging sexual harassment. More than two dozen women, including former Fox News anchor Megyn Kelly, recounted specific claims of sexual harassment to investigators, which ultimately helped cement Fox's decision to dismiss Ailes last July. Carlson's lawsuit was later withdrawn after Fox agreed to pay her $20 million

More than 50 companies have said they're suspending ads from the popular primetime show in the wake of the O'Reilly controversy. Most have moved their advertisements to Fox News' other programming, meaning that the network still gets to keep most of the ad dollars. In their place, commercial breaks during the "O'Reilly Factor" have been featuring mostly B-list advertisers. 

O'Reilly is Fox's top revenue producer, generating about $446 million in ad sales from 2014 through 2016, according to the New York Times report.

As of Monday morning, Bayer (BAYRY) ,  ConAgra Foods (CAG - Get Report) ,  Pfizer (PFE - Get Report) , Mercedes Benz, Subaru,  AllState (ALL - Get Report) and a slew of other major U.S. brands said they had pulled advertising from the "O'Reilly Factor." A number of high-profile advertisers continued to run advertisements during the show, including  Comcast (CMCSA - Get Report) , Microsoft (MSFT - Get Report) and AT&T (T - Get Report) , among others.

"Based on the serious nature of these allegations, we have made the decision to suspend all advertising on the program in question," a Bayer spokesperson said in a statement on Monday. "This suspension has been requested immediately and will be handled as expeditiously as possible."

The extensive advertiser boycott and the ongoing controversy have heightened analysts' concerns about Fox's pending $14.2 billion purchase of satellite TV and Internet provider Sky plc (SKYAY) . The European Commission on Friday approved the Sky acquisition, but now Fox must obtain a license from Britain's broadcast and communications regulator, Ofcom, of which Fox could face scrutiny from based on the regulators' "fit and proper" rule.

The rule weighs whether a company's management is "fit and proper" to hold the license, and if O'Reilly stays on at Fox, it could cause regulators to question whether management is "overly tolerant" of inappropriate workplace environments, according to Pivotal Research analyst Brian Weiser. 

O'Reilly himself hasn't yet addressed the allegations on-air, but he did issue a statement last week denying that any claims had been filed against him during his 20 years at Fox. 

The controversy may have caused advertisers to flock in droves, but so far, it has done little to turn off O'Reilly's loyal fan base. The "O'Reilly Factor" drew 3.7 million and 3.8 million viewers last Monday and Tuesday, respectively, an increase of 14% from the same two nights in the week prior, according to  USA Today

And President Donald Trump even came to the defense of O'Reilly last week, saying in an interview with the Times that he didn't think the host "did anything wrong" and that O'Reilly shouldn't have settled the lawsuits. 

Viewers' continued loyalty to O'Reilly amid the accusations has caused several commentators to question whether fans of the show pay much attention to controversies such as these. New York Times media columnist Jim Rutenberg wrote this week that O'Reilly has long gained viewers' trust by pledging to be a source of truth among an increasingly progressive media landscape, which serves as the basis for why some viewers remain inherently skeptical about the accusations. 

Others pointed to how Trump faced numerous allegations of sexual assault and was still able to secure the presidential nomination as evidence for how many prominent male figures are able to withstand controversies related to sexual assault or harassment. 

Viewers may not be the only faction that proves to be resilient, either. Advertiser boycotts can become difficult to sustain if large numbers of viewers continue to keep watching the programs that brands are blacklisting,  The Washington Post noted

Similarly, talk radio host Rush Limbaugh lost a slew of advertisers after he called Georgetown law student Sandra Fluke "a slut" back in 2012. It didn't take very long before those brands resumed advertising on the show, however. Radio distributor iHeartMedia said last August that it had renewed its contract with Limbaugh for an undisclosed term.