It was a concerned parent who helped investigators bust the Fairfax County gang members accused of prostituting high school girls for years, authorities said.

Child sex-trafficking charges were unsealed Thursday against five members of the Underground Gangster Crips, a Fairfax-based subset of the Crips gang. The gang members allegedly pimped teenage girls and used violence if the teens tried to quit.

Fairfax County police began investigating the case in November, Chief David Rohrer said.

The police department learned of the case after a victim's parent was suspicious of the girl's activities and tracked her whereabouts. The parent finally had a conversation with the girl, "figured out what was going on" and called police, said Virginia Attorney General Ken Cuccinelli.

"A vigilant parent helped make this case," Cuccinelli said.

Officials urged parents to stay abreast of their child's activities -- at school and online -- and ask questions if something doesn't seem right.

"We need to be more aware of the risks that our sons and daughters face," Rohrer said.

-- Emily Babay