Frederick is a city in, and the county seat of, Frederick County in the U.S. state of Maryland. It is part of the Baltimore–Washington Metropolitan Area. Frederick has long been an important crossroads, located at the intersection of a major north–south Indian trail and east–west routes to the Chesapeake Bay, both at Baltimore and what became Washington, D.C. and across the Appalachian mountains to the Ohio River watershed. It is a part of the Washington-Arlington-Alexandria, DC-VA-MD-WV Metropolitan Statistical Area, which is part of a greater Washington-Baltimore-Arlington, DC-MD-VA-WV-PA Combined Statistical Area. The city's population was 65,239 people at the 2010 United States Census, making it the second-largest incorporated city in Maryland, behind Baltimore. Frederick is home to Frederick Municipal Airport (IATA: FDK), which accommodates general aviation, and to the county's largest employer U.S. Army's Fort Detrick bioscience/communications research installation.