She was a prostitute, whose death was the result of a heroin overdose and a blow to the head.
Jessica is found dead from a heroin overdose and a blow to the head. As her body is examined, it's found that an unknown person had performed chest compressions postmortem which cracked a rib. Her body had then been repositioned, causing blood to flow from her nose.
Deb is taken by surprised when she discovers that Thomas Matthews had sent flowers to Jessica on the day of her death. He asks to have dinner with Deb and admits to being the one who performed CPR on Jessica, but he couldn't revive her. Matthews pressures Deb to keep his involvement a secret and close the case to protect him. She reluctantly agrees because he had helped her so much through the years.
LaGuerta then turns in Matthews herself to earn a promotion. Matthews blames Deb, regardless.