WASHINGTON, Nov 27 - A false report of an active shooter at the Walter Reed military hospital outside Washington led people to take shelter on Tuesday, before an all-clear was given and the U.S. Navy confirmed the incident was related to a drill.In preparations for the drill, a mass notification alert was mistakenly sent to people at Naval Support Activity Bethesda, the military base that houses the medical center, the Navy said in a statement.The notification did not contain the words "exercise" or "drill," which prompted workers to respond as if an active shooter actually was at the facility, the Navy said.Police swept at least one building after receiving the report about a possible shooter around 2:23 p.m. ET (1923 GMT).People at the medical center, including a U.S. congressman, sheltered in place for at least 30 minutes, while the base urged others to find cover inside vehicles or buildings as a precaution."I am currently at Walter Reed Medical in Bethesda where we've been told there is an active shooter," U.S. Representative Dutch Ruppersberger wrote on Twitter at one point. "I am currently safe in a conference room w/ approx 40 others."The base gave the "all clear" around 3:20 p.m. It said in a Facebook post that no active shooter was found.Walter Reed National Military Medical Center is the largest joint military hospital in the United States, specializing in care for veterans of the U.S. Navy and Army and their families. It provides care to more than 1 million beneficiaries per year.