PORTLAND, Maine — An Auburn man and a Lewiston woman arrested New Year’s Eve in Bangor and charged with prostituting a 13-year-old girl pleaded guilty Wednesday in U.S. District Court to transporting an individual in interstate commerce to engage in prostitution and aiding and abetting.

By pleading guilty, Alvin “Chip” Houston, 27, of Auburn and Shawna Calhoun, 24, of Lewiston admitted to arranging for the girl to have sex with adults through advertising pay-for-sex services via the free classifieds website Backpage.com, according to documents filed in federal court in Portland.

They were arrested Dec. 31 at an Odlin Road motel and charged in state court with sex trafficking. Those charges were dismissed when Houston and Calhoun were charged in federal court.

Both have been held without bail since their arrests.

An investigation into the whereabouts of a 13-year-old Lisbon girl who reportedly ran away from home Dec. 21 led to the arrest of Houston and Calhoun, according to court documents.

The teen contacted Calhoun on Facebook looking for contact information for another individual, according to the prosecution version of events to which she pleaded guilty. The girl told Calhoun she was 15 going on 16.

The girl told investigators that she had “engaged in prostitution” in Portland and Boston, the affidavit filed in federal court in Portland said.

The ad, copies of which were included in the affidavit filed Jan. 2 at the Penobscot Judicial Center, was for a 21-year-old “Spanish Doll” available for outcalls in the Bangor area.

The girl was with the pair when FBI special agents and Bangor police conducted a sting at the hotel on New Year’s Eve, according to a previously published report.

The teenager, who was not arrested or charged, was released to her mother about 5:30 a.m. Jan. 1, according to Bangor police.

Calhoun and Houston face up to 10 years in prison and a fine of up to $250,000.

Sentencing dates have not been set.