A Pennsylvania man pleaded guilty Thursday to sex trafficking four teenage runaways from Ohio who were found in his car — along with 8,000 individual dose bags of heroin — during a traffic stop last year.

Robert Middlebrook, 41, of Clairton, will likely face an agreed-upon 12-year federal prison sentence when he returns to court in Pittsburgh on May 9.

Middlebrook's co-defendant, Kiari Day, 26, of Rankin, pleaded guilty in November to helping Middlebrook by teaching the girls how to interact with "johns" and helping post online prostitution ads. She'll be sentenced in December.

Day and Middlebrook were indicted in August after federal authorities took over what began as a state police traffic stop in Lower Paxton Township, near Harrisburg, on Feb. 17.

Middlebrook was on probation for heroin possession when troopers charged him with drug offenses and trafficking in minors — three of the girls were 17, one was 16. But those charges were dropped after federal investigators took over the case as part of an emphasis on human trafficking started two years ago by U.S. Attorney David Hickton in Pittsburgh.

On Thursday, Assistant U.S. Attorney Jessica Lieber Smolar told U.S. District Judge Arthur Schwab that Middlebrook and Day brought the girls to Pennsylvania.

Middlebrook drove two girls to the Pittsburgh area on Feb. 9, and returned with Day two days later to bring back the other two. The girls were from the Canton-Akron area, about two hours west of Pittsburgh. The girls were all friends and runaways, and one girl that Middlebrook met last January convinced her friends to go with him and Day.

Once in Pennsylvania, Middlebrook told the girls they'd work as prostitutes through ads placed on Backpage.com, Smolar said.

Middlebrook and Day had the girls pose in lingerie at his home for cellphone photos used in the ads. Day helped the girls style their hair and taught them how to act with "johns" — or male sex customers.

The girls were also given marijuana and ecstasy and supplied with condoms, Smolar said.

Smolar told the judge that Middlebrook knew how old the girls were because he took them to a body piercing parlor and had to give permission for them because they were all under 18.

Middlebrook allegedly let his friends have sex with the girls and claimed he had paid someone to kill them if they didn't perform sexually.

Middlebrook and Day rented hotel rooms in Pittsburgh's South Hills suburbs in February, where the girls were made to wait for sex customers.

At one point, the girls texted Middlebrook, saying they "wanted to go home" and were hungry and without food, but were ignored, Smolar told the judge.

Middlebrook's defense attorney, Douglas Sughrue, declined comment after the guilty plea.

Schwab still must approve the 12-year sentence. Under the sex-trafficking statute, Middlebrook can be given any sentence between 10 years and life in prison.