Texas American Federation of Teachers filed a federal lawsuit Tuesday against the Texas Education Agency and its commissioner, Mike Morath, alleging the state unlawfully investigated educators for protected speech following the assassination of conservative leader Charlie Kirk on a college campus in Utah in Sept.

A teachers union in Texas sued the state’s education agency Tuesday over its investigations into educators who criticized conservative activist Charlie Kirk following his death.

According to the union’s complaint, filed Tuesday in Texas federal court, the state action “quickly unleashed a wave of retaliation and disciplinary actions against teachers based on their First Amend...