TAMPA, Fla. (AP) — A Florida woman has pleaded guilty to mailing threatening letters containing white powder to elementary schools where she previously worked as a teacher.

Maria Bassi Lauro, 66, of Davenport, faces up to 15 years in prison after pleading guilty Thursday in Tampa federal court to three counts of making hoaxes that appeared to involve a biological agent or toxin, according to court records.

In August and September of 2018, Lauro sent threatening mailings to employees at four central Florida elementary schools where she had previously worked as a teacher, prosecutors said. All four contained a suspicious powder, and three included notes indicating they were “punishment,” officials said. Tests later showed it was baking soda.

Lauro had effectively been fired for poor performance from each of the schools, and each victim who received a note had played a role in her performance reviews or terminations, officials said. Lauro admitted in her plea agreement that she had sent the mailings because she was upset and wanted the victims to believe they had been exposed to a deadly biological toxin.