Alan Richter is the lover of serial rapist and murderer Thomas Mathers, despite Mathers hating homosexuality and stalking and raping women out of never having a girlfriend.
Alan answered the door when detectives arrived to question Mathers. There were pictures of the two men together, and in real time, both were just coming out of the shower. Mathers was obviously spiteful and disgusted when Alan openly talked about their relationship and spoke affectionately to Mathers. ME Warner arrived to take a cheek swab from Mathers to tie to DNA at the scene of one of the rapes, but it didn't match.
Mathers was later proven guilty and arrested when it was only his blood matched from a transfusion he had after leukemia. (SVU: "Weak")