Armstrong joined ProPublica in March, 2018, where he is a senior reporter.
He authored two articles for The New Yorker exposing the work of expert witnesses in child abuse cases. The first, about a renowned scientist who promoted his own theory about how infants with broken bones were injured, can be found here and here. For several weeks, it was among the most read stories on The New Yorker Web site. Another article featured the work of a former leader of the anti-vaccination movement and his pivot to defending parents accused of child abuse. It can be read here and here.