Richard Byrne first saw the posters in his Washington, D.C., neighborhood on Thursday morning as he walked to a commuter train station.

The text-heavy fliers, plastered on streetlight poles and at bus stops, urged residents to report “illegal aliens” to the federal authorities. At first glance, the posters looked official. They included the logo of the Immigration and Customs Enforcement agency and cited federal laws against harboring people who entered the country illegally.

But Mr. Byrne, a 51-year-old editor and playwright, was not fooled. “I saw six or seven of these fliers in the space of a block and a half, which is extraordinarily unusual,” he said in a telephone interview.

Within hours, ICE had disavowed the posters. Residents were removing them, and officials were criticizing the people — still unidentified — who put them up.