The Transportation Security Administration (TSA) is responsible for protecting the Nation's transportation systems to ensure freedom of movement for people and commerce. As part of its mission, TSA screens checked baggage to deter, detect, and prevent the carriage of any prohibited items, such as explosives and incendiaries, onboard passenger commercial aircraft. TSA primarily relies on its screening workforce and two types of equipment to screen checked baggage - explosives detection systems and explosives trace detection. Through covert testing conducted at domestic airports, we determined whether Transportation Security Officers were following established policies and procedures to prevent threat items from being placed onto commercial aircraft. We also determined the operational effectiveness of TSA's checked baggage screening technology. We identified vulnerabilities in this area caused by human and technology-based failures.