LYNN – An online ad attracted unwarranted, but perhaps helpful, attention for a teenage sex worker after she allegedly arranged a date with a cop. Meanwhile, the state attorney general has urged a federal case against the ad service for exploiting underage girls.Lynn Police said Monday that detectives arrested a 17-year-old female for sexual conduct for a fee on Sunday night. The arrest stemmed from detectives monitoring the escort section of the Internet website Backpage.com, which is known for advertising commercial sex.The detective noticed an ad that featured a woman in various stages of undress and listed a contact phone number. The detective began a string of text messages and eventually spoke with the suspect via phone. The two agreed to meet at a specific location and the female was arrested on her arrival.In February, Attorney General Maura Healey filed a brief urging the federal court to go forward in a case against Backpage.com that alleges the website knowingly and actively supports human trafficking. The case was filed by three women who were sold for sex on Backpage.com when they were as young as 15 years old.”Websites that actively facilitate human trafficking should be held liable for this serious and widespread problem in the Commonwealth,” Healey said in a press release announcing her filing of the brief. “Backpage is known for advertising commercial sex, and its recent growth and dominant position in the market call into question its supposed efforts to curb prostitution and child exploitation.”In the federal case, Doe v. Backpage.com LLC, the plaintiffs allege they were recruited by sex traffickers, advertised on the website and then repeatedly sold for sex in various locations across Massachusetts. The plaintiffs further allege Backpage promotes sex trafficking by fostering the online market for illegal commercial sex and by helping traffickers develop effective ads and evade prosecution. The complaint also alleges Backpage.com knows that many of the trafficking victims on the website are underaged girls.Healey’s brief argues that because Backpage.com does more than simply allow users to post on its website, the company violates the Massachusetts Anti-Trafficking Law and Chapter 93A, a state law prohibiting unfair or deceptive business practices.Backpage.com has requested the court dismiss the case, arguing that it is immune from liability under the Communications Decency Act (CDA) for content posted by third parties.