YES IT IS ILLEGAL. USC 1466A, which specifically says 'cartoon' images:
https://www.law.cornell.edu/uscode/text/18/1466Apeople HAVE been charged with just cartoon pornography, i've found local US news sites where bitter ex-wives turned their husbands in. 100% manga, no 3D shit, it counts.
don't believe the naive weeaboos who say it's not a law, or it's never been applied, or it's only been used in conjunction with photographs. people have gone to jail purely because of lolicon by now.
the law itself is pretty bullshit, and if you're worried, here are the guys you want to speak to:
https://cbldf.org/they have some success stories, like this guy:
https://www.youtube.com/watch?v=ltdgSEPPCZsarrested in an airport because of a stupid chibi meme image.
pic unrelated. Alan Moore is an artiste, so this counts as art.