LOS ANGELES — YouTube CEO Susan Wojcicki was blunt about the problem. "Bad actors" are exploiting the YouTube system. The Google-owned company would hire more human reviewers to thwart those who try to "mislead, manipulate, harass or even harm."

Spend a little time with YouTube and videos that kids might come across, and you'll see what she means.

Just hours after Wojcicki’s late Monday blog post unveiling plans to increase the number of people reviewing YouTube’s videos to 10,000 next year, a 25% increase, USA TODAY found dozens of videos that appeared to slip through site filters designed to keep potentially harmful, explicit content away from kids and young teens.

One trouble spot: a longstanding genre called "YouTube Poop,” where people take an existing, popular cartoon, like Nickelodeon's SpongeBob SquarePants or Peppa Pig, and record a new soundtrack with explicit sexual or violent themes.

Some we found were age restricted, meaning they could only be viewed by signing in with a Google account, which is only available to those of 13 and older. If YouTube had identified the videos as adults-only, they should have barred anyone under 18 from viewing.

But others in this genre, with titles like "Joker has trouble braking his c--- s--ing habit," based on the Batman nemesis, “Cedric Rapes Harry,” using footage from one of the films based on the children's novels, and “YTP Peppa Pig Plays Sexual Games” did not carry any age restriction prompt — meaning users of any age could watch them. One of the worst offenders was a ripoff of the "Let it Go" song from Frozen, with the words changed to sing crudely about a sexual act. This video had nearly 2 million views.

When contacted by USA TODAY, Walt Disney Co., owner of Frozen, said in a statement: “We have expressed our deep concern to YouTube over these unauthorized and inappropriate videos that misuse our characters and are working with them closely on this matter."

Warner Bros., which owns the Harry Potter and Batman footage, declined to comment. Nickelodeon didn't respond to a comment request.

In the last few months, media critics and child-welfare advocates have assailed YouTube, the world's largest video network, for allowing thousands of videos using kids-themed content — or involving kids — that contain violence, were sexually explicit, or exploit children.

Amid the backlash, YouTube has rolled out a series of changes, from tightening filters, yanking videos and promising to beef up enforcement.

Just before Thanksgiving, YouTube said in a blog post that it had recently implemented policies to age-restrict (only available to people over 18 and logged in) "content with family entertainment characters but containing mature themes or adult humor."

After USA TODAY contacted YouTube about the adult-themed cartoons that weren't flagged as age restricted, the videos were taken down or age-restricted.

The company on Tuesday said it will impose those age restrictions on adult-themed kids content when “when flagged.” Given the number of videos, it's trying to use machine learning to catch more.

"Content that misleads or endangers children is unacceptable to us," it said in a statement to USA TODAY.

Reliance on users and algorithms to alert YouTube to dangerous content has repeatedly come under fire in the last year, perhaps Wojcicki's most daunting challenge to her leadership since being named CEO in February, 2014.

That objectionable videos only get removed after they've been flagged, is "not acceptable," Josh Golin, the executive director for advocacy group Campaign for a Commercial Free Childhood. "My daughter has already seen it."

400 hours of video uploaded per minute

Like other big tech companies such as Facebook, use of automated networks that require minimal human oversight have fattened profit margins for YouTube and its parent Google. And the network, which receives 400 hours of new videos per minute, says increasingly sophisticated machine learning is necessary to handle the scale of video uploaded every day. Investing in the technology will make a YouTube a safer place, it says.

But its current technological safeguards haven't been fool-proof, creating the risk that YouTube gets a reputation for the kind of content mainstream advertisers would disavow.

Advertisers last month pulled away from the service after news reports showed child predators using videos of young children as de facto pedophile chat rooms. Wojcicki, in her blog post, called 2017 a "very tough year."

The reports of child predators followed earlier reports of scary content seeping into YouTube Kids, started two years ago a stand-alone mobile app. Critics have seized on YouTube for allowing some of the non-kid friendly content from appearing in there as well. But YouTube claims than less than .005% of videos viewed in the Kids app were removed for being inappropriate. And in our tests this week, USA TODAY found You Tube Kids to be mostly what's been advertised—a safer and more wholesome place. The lone offender: a video of an elephant killing a security guard. (It's since been taken down by YouTube.)

Some say nothing will change without altering the incentive for creators.

YouTube splits ad revenues 55%-45% with the creator, who gets rewarded for the clicks. The more eyeballs, the greater the checks. According to Forbes, the 5 YouTubers with the largest audiences earned a collective $43.5 million in 2016.

And to discourage the video makers who slip sex and violence tags into what seems like kid themed videos, in search of more views, YouTube says offenders will be immediately de-monetized, meaning no profits for them.

Kids programming is hot on YouTube

In the past two years, a new genre of programming on YouTube has emerged, focusing on even younger children: the 3- to 6-year-old set. Now, those kids channels are the most popular genre on YouTube, despite YouTube's terms of service that bans under 13-year-olds from watching.

Four of the top five YouTube channels in the U.S. are kids-related, according to measurement service Tubular Labs, highlighted by Ryan Toy's Review (719 million views in October) in which 5-year-old kids reviews toys, or ToyScouter (457 million views), featuring kids singing nursery rhymes.

These channels are clean. It's the sub-genres, like "Poop" where YouTube gets into trouble. A youngster might stumble upon a sexualized video of Mickey Mouse if his viewing history contains Mickey Mouse and the adult-themed cartoon hasn’t been flagged as age restricted.

"These kinds of mash ups seem funny and ironic to adults who create them, but to a child they can be disturbing and confusing," says Michael Rich, an associate professor of pediatrics at Harvard University.

In her blog post, Wojcicki insisted that investing in machine learning is the way forward, but human reviewers are necessary for "making contextualized decisions on content."

"Since we started using machine learning to flag violent and extremist content in June, the technology has reviewed and flagged content that would have taken 180,000 people working 40 hours a week to assess," she said.

The sad truth, according to Corrine O'Neill, a Canadian animator who produces children's cartoons for her WildCanadaKids YouTube channel, is that content with sex, violence and other edgy tags will always rise to the top and get more viewers.

"The YouTube algorithm is a chum factory," she says, referring to "chum" as slang for click bait. "It rewards quantity over quality. It's YouTube promoting these videos that brought them to people."