In January 2017, Ekwonna allegedly met with the 15-year-old. While she was in the back seat of his car, he pointed a gun at her and demanded she return the money he had just paid her, according to the U.S. attorney's office.
Ekwonna also used his Tagged account to reach out to many other users to solicit them for sex.
He faces up to life in prison if convicted.
Ekwonna was fired from the D.C. police department in April after he was initially charged. He had been with the department for 14 months.
D.C. Police Chief Peter Newsham said the department was reviewing the circumstances under which it hired Ekwonna, who had previously been with the D.C. Department of Corrections and was facing a lawsuit at the time by an inmate who accused Ekwonna of beating him.
D.C. settled that claim for $20,000 in February.