MONTGOMERY, Ala. - An Alabama man who was accused of posing online as pop star Justin Bieber while urging young girls to send him explicit photos of themselves pleaded guilty Thursday to producing child pornography, authorities announced.

Christopher Patrick Gunn, 31, of Montgomery, promised young girls free tickets and backstage passes to a Bieber concert if they would agree to send him inappropriate photos or webcam transmission of themselves, authorities said.

A statement released Thursday by U.S. Attorney George L. Beck in Montgomery said Gunn used chat rooms and other social media outlets to threaten hundreds of girls ages 9 to 16 throughout the United States and abroad over a period of more than two years as part of a far-ranging "online sextortion scheme" that included posing as Bieber and other ruses.

"These child predators irreparably damage the children they victimize," Beck said in the news release. "The scope of this defendant's exploitation of children is staggering; he not only preyed upon children in our community, but in communities across the United States."

Beck's statement said Gunn's plea included two counts of making child pornography, 15 counts of interstate extortion and seven counts of internet stalking.

Gunn also was accused of working another scam in which he posed as the new kid in town on social media sites such as Facebook, and once he gained the girls' trust through fake social media profiles he would solicit embarrassing information about them, according to Beck's statement. Gunn would ask for explicit photographs and threaten to make available the embarrassing information if the girls refused, it said.

Authorities allege that Gunn's actions took place from 2009 until 2012. On March 20, 2012, FBI agents executed a federal search warrant on Gunn's house. They said they seized a cellphone and a laptop computer containing images and videos of child pornography.

Beck's statement said Gunn had obtained explicit photos of young girls in numerous states, including Alabama, Mississippi and Louisiana.

The Federal Bureau of Investigation said it is continuing to work to identify any additional victims in other states including Florida, North Carolina, Virginia, Pennsylvania, Michigan and California.

Based on his guilty plea, Gunn faces no less than 25 years and as much as 35 years in federal prison, Beck's statement said. It added that he could be ordered to spend the rest of his life on supervised release.