A convicted murderer admitted Monday that he had sexually assaulted a 2-year-old boy in Camden in August 2012, a month before he sexually assaulted a 12-year-old girl and killed her 6-year-old brother.

Osvaldo Rivera, 34, formerly of Camden, pleaded guilty to aggravated sexual assault in Superior Court for the attack on the 2-year-old. He faces a 15-year prison sentence under his plea deal, the Camden County Prosecutor's Office said in a news release.

While looking into the September 2012 murder of the 6-year-old and assault on the 12-year-old, investigators used a cheek swab to run Rivera's DNA through the New Jersey State Police DNA database.

That system flagged a possible match with DNA evidence from the assault on the 2-year-old, the Prosecutor's Office said. A follow-up test comparing Rivera's DNA sample with the evidence verified the match.

Rivera's 15-year term would run concurrently with a 110-year prison sentence he is serving for the attack on Dominick Andujar and his sister.

Rivera assaulted the girl on the night of her 12th birthday in her family home on Ware Street and cut her with a butcher knife. Her wounds included deep gashes across her throat, the Prosecutor's Office said.

Dominick was awakened by screams and rushed to his sister's defense. He fought Rivera, who slashed his throat.

Dominick was widely hailed as a hero. As he fought, his sister was able to escape the house and get help from a neighbor.

Two other sisters were asleep in the house. Their mother, who has kidney problems, had gone to the hospital that night after feeling ill.

At that trial, Rivera rejected a plea deal. He was convicted in July 2014. Superior Court Judge Michele Fox noted at the time that he had expressed no remorse.

Rivera is scheduled to be sentenced April 17.

Asked about the concurrent sentences, Christine Shah, the assistant prosecutor who tried both cases against Rivera, said it was important to conclude the case.

"Victims and their families are entitled to a final resolution," she said in a statement. "Rivera had to be held accountable for the crime he committed against this toddler, regardless of having been previously sentenced to life in prison in another case."