Washington ? Kimberly Hall was twice betrayed by men she met dating online. Both turned out to be married.

So she started doing background checks on her dates using a Web site called Intelius. Now, the 33-year-old from Laurel, Md., is engaged to a man she met on Blackplanet.com, but even he had to undergo record checks.

“He wasn’t happy” about doing it, Hall said of her fiance. But eventually he turned over his Social Security number.

In the past decade, sites such as Yahoo Personals, Match.com and eHarmony helped make Web-based courtship mainstream for 10 million current daters. But some seasoned veterans say the thrill of using the Internet’s power to find soul mates has given way to caution. Singles now draw on a growing arsenal of security and research tools – from services that verify identity and background to companies that provide temporary phone numbers as a barrier to stalkers. Sites like DontDateHimGirl.com allow scorned lovers to warn others away from their bad dates.

The growth of the dating-security industry is part of the evolution of the Internet, where every powerful tool such as online banking or e-mail comes with a dark side of data theft and spam messages. MySpace, which started as a way for kids to exchange likes and dislikes, recently set up checks against sex-offender registries. By comparison, dating sites have been slow to adopt safety-filtering measures; few dating sites conduct their own background checks on members and only one seeks to verify marital status.

Thirty-one percent of American adults say they know someone who has used a dating Web site, and nearly 60 percent of Internet users said they think a lot of online daters lie about their marital status, according to Pew Internet & American Life Project study last year.

Dating site True.com is the only major Web firm that conducts criminal and marital background checks on all of its members – a practice that keeps 2 percent of applicants from joining because they are convicted felons. Three percent flunk because they are married, the company said.

The company sued a felon who slipped through the background-check process. Herb Vest, chief executive of True.com, said he won’t rule out suing a married person who gets onto the site, either.

“It’s emotional harm. When you think of it, if you date someone … you fall in love with them and find out they’re married, it’s heartbreaking,” Vest said.

One 56-year-old Texas woman discovered how dangerous it can be to try to navigate online dating without a security filter. She dated a man she met on Yahoo Personals for eight months before a simple Google search revealed he was convicted of murder and insurance fraud.

“It was really very scary,” said the woman, who spoke on condition of anonymity for safety reasons. “It’s very important to do a background check whenever you go out with someone, even if you have to pay” for it, she said. “You really cannot be too safe.”

But marketing security and screening features on dating sites can cut both ways for the business.

“No dating site wants to admit there’s bad people on their site,” said Dave Evans, a consultant to the online dating industry who blogs about it on OnlineDatingPost.com. Offering screening or other security add-ons might suggest to users, ” ‘you better pay this or you could get raped or dismembered,’ ” he said, which “is not an inviting come-on for new members.”

Some daters turn to College Park, Md.-based Trufina, which verifies someone’s age, identity and address – a service it began offering free last week, figuring that will draw customers willing to pay an additional $9.95 for criminal checks. Several other background-check firms, like Intelius, which built their businesses doing criminal checks for employers, are increasingly finding customers interested in researching their dates.

There are also other companies like Jangl, a start-up that offers free temporary phone numbers for eBay customers and dating singles who, at least initially, might want to keep their real phone numbers to themselves. Another company, Tossable Digits, offers a similar service for a fee.

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One site promises one-stop shopping for dating researchers: DontDateHimGirl.com is a repository of 18,000 men whose former girlfriends allege they are cheaters, liars or two-timers. The database, started in 2005, comes complete with comments from the accusers as well as responses from the accused.

“Do not let the smile fool you,” says an anonymous post next to a photo of a handsome, shirtless man wearing boxing gear. “This man is the king of cheaters … He has never been fait(h)ful to ANY of the women he was in a relationship,” his accuser wrote. “STAY AWAY FROM HIM!”