A third teenager has been charged with the shooting death of District school principal Brian Betts in what authorities are calling a robbery-murder set up through online and phone sex chats.

Dontra Q. Gray, 18, of Oxon Hill, is accused of first-degree murder, armed robbery and conspiracy. Montgomery County County police said Gray and two other teens got themselves invited to Betts’ Silver Spring home after a brief chat through Adam4Adam, a Web site catering to gay men seeking romance. Betts’ body was found April 15 after the 42-year-old didn’t show up to work at the Shaw Middle School at Garnet-Patterson. He had been shot in the back, but there was no sign of forced entry.