Have the mechanics been ruled out as the culprits? If so, the article should have mentioned it.
Fitzpatrick said the offensive materials “consisted almost entirely of inappropriate business cards” found mostly on the Embraer E-90s flights to Las Vegas. The most recent reported incident was in February of this year, Fitzpatrick said.
http://www.nydailynews.com/news/national/las-vegas-officials-clean-x-rated-litter-article-1.1155058