Management of our critical infrastructures is a vital component of our economic and homeland security policy. The degree to which these infrastructures are interdependent, due to increasing system complexity and technology, further complicates the task of infrastructure managers to maintain service. In this paper we use economic models of the U.S. economy to identify hidden interdependencies in the supply chains of infrastructure. Critical connections between infrastructure systems, whether direct or indirect, pose a risk of one disruption causing a ripple effect across the economy. Our analysis shows that in many cases, the highest interdependencies between critical infrastructures occur upstream (e.g., in the second or third-level of the supply chain). Specific results are shown for the large upstream interdependencies (up to 10 times the direct dependencies) between transportation and power generation sectors. By revealing these upstream interdependencies infrastructure managers can take further preventative measures or make additional investments to avoid future infrastructure disruptions.