Florida ? Police in the 1970s urged citizens to “drop a dime” in a pay phone to report crimes anonymously. Now in an increasing number of cities, tipsters are being invited to use their thumbs – to identify criminals using text messages.

Police hope the idea helps recruit teens and 20-somethings who wouldn’t normally dial a Crime Stoppers hot line to share information with authorities.

“If somebody hears Johnny is going to bring a gun to school, hopefully they’ll text that in,” said Sgt. Brian Bernardi of the Louisville, Ky., Metro Police Department, which rolled out its text-message tip line in June.

Departments in Boston and Cincinnati started accepting anonymous text tips about a year ago. Since then, more than 100 communities have taken similar steps or plan to do so. The Internet-based systems route messages through a server that encrypts cell phone numbers before they get to police, making tips virtually impossible to track.