Chicago Public Schools prevented federal officers from from going into an elementary school on Chicago’s Southwest Side Friday and talking to students, according to school officials.

The U.S. Secret Service said its agents visited a Chicago elementary school Friday while investigating a threat, hours after school officials mistakenly claimed that U.S. Immigration and Customs Enforcement agents had come to the building.

Fearing federal agents asking to search a Chicago elementary school Friday morning were from Immigration and Customs Enforcement, school officials denied them entry. But it turned out they were US Secret Service agents pursuing an investigation.