Maggie Lambert is the overbearing mother of serial killer Trace Lambert. She always found him a "loser" and was absolutely disapproving of his work in a traveling carnival. Lambert would frequently visit his mother in her house when he was in town as part of his schedule. When in New York City, Lambert would also kidnap low-class women in sex work or vagrancy, raping, torturing, and murdering them all before preserving their remains at his hideout, starting with his ex-girlfriend Meredith Hart.
When Meredith's son with autism, Jack, recalled the license plate, it was plenty to track Lambert down to Maggie's house. She said Lambert wasn't there, but he walked out and surrendered to the detectives. Maggie was brought along and offered coffee, where she bitterly condescended Lambert to the detectives. This was used to interrogate Lambert, which resulting in him being mad enough to confess to all his crimes and insist Maggie knew everything. (SVU: "They'd Already Disappeared")