A Cobb County man has pleaded guilty in what federal prosecutors call one of the most egregious “sextortion” cases in metro history.

Tremain Hutchinson is to be sentenced Dec. 30 for using social media to gain the trust of teenage girls and then enticing or coercing them into transmitting nude photos and video of themselves. Hutchinson also sexually assaulted at least three young victims, prosecutors say.

Hutchinson, 28, pleaded guilty Thursday to five of the 24 counts against him. Because Hutchinson did not enter into a plea agreement, federal prosecutors can still try him on the remaining 19 counts. That decision will likely be determined by Hutchinson’s upcoming prison sentence, which is expected to be severe.

Hutchinson befriended young girls on social media sites, pretending to be a teenager and using aliases such as “Quan,” “Money” or “Mario.” Eventually, he exchanged texts with the girls on their cellphones and got them to send sexually explicit images, prosecutors said.

U.S. Attorney Sally Yates has previously said the charges against Hutchinson were “at the outer boundary” of other “sextortion” cases. These cases typically target offenders who lure young victims into sending sexually explicit photos and video but do not involve predators who go find their victims and sexually assault them.

On Sunday, Yates called Hutchinson’s plea a “positive step” in resolving the case. “We are continuing to work with the victims to ensure that the defendant receives a sentence commensurate with his horrific crimes,” she said.

DeKalb County police launched the initial investigation against Hutchinson after a woman reported she found graphic images on her 15-year-old daughter’s cellphone. One included her daughter engaging in a sex act with her 13-year-old brother, DeKalb police Det. Torrey Kennedy testified earlier this year.

Hutchinson had persuaded the 15-year-old girl to initially send him nude photos of herself and he then threatened to post them on social media or her school’s website if she did not send him more sexually graphic videos, such as the one with her and her brother, Kennedy said.

The same time DeKalb police were investigating Hutchinson, Forest Park detectives had begun their own, unrelated investigation of him, according to court records.

Posing as a 16-year-old boy named Quan on the social media site Tagged, Hutchinson befriended young girls in Forest Park and drove to visit them, according to court testimony. He raped one 16-year-old girl after she had let him know she was home sick from school and also raped a 13-year-old girl and an 11-year-old girl from Forest Park, federal prosecutors said.

In his guilty plea on Thursday, Hutchinson admitted that, from November 2011 to April 2012, he got young girls to send him explicit photos of themselves and that he send graphic photos of himself as well. He also pleaded guilty to raping one victim and committing statutory rape and child molestation against another young victim during that time period.

Prosecutors allege Hutchinson committed some of his “sextortion” crimes while he lived with his mother at her Mableton apartment, where he slept on the sofa.