he became sexually aggressive. and i tried to fight him off. and at that point i knew i was in trouble. reporter: dustin trickett is a convicted sex offender. she only found that out after the attack. i was basically prey for him. reporter: jeffrey marsalis. dozens of women met him on match.com but he turned out to be a convicted serial rapist who found his prey online. there are more than 1,500 online dating sites and over 20 million americans use them. that s double the amount of users from just five years ago. so there s a growing demand for online detectives to suss out who s looking for love and who s looking for victims. my match checker really comes into play to add a little bit more of the trust but verify idea behind online dating. reporter: robert buchholz runs mymatchchecker.com. at 9.95 per search, they ll run a background check on anyone you meet online. some of the things that we can find really is from misdemeanors to felonies to a listing on the national s
reporter: true.com, a dating site that requires a background check before users can become active. if you want to check up on someone while you re away from your computer, maybe on that first date, pull out your smartphone. there s an app for that. like date check. type in a name and state and for $14.95 get a criminal background check, property search, even social network profile. but a background check doesn t put a potential match completely in the clear. we feel like there s a lot of inaccuracies and incomplete data in these databases and that they provide a false sense of security. reporter: for rebecca, even a little bit of information about her attacker would have been welcomed. if somebody seems too good to be true they probably are. reporter: for abc news, i m becky worley.