The envelopes were addressed to 12 university rectors across Greece, from its islands to the mainland and the capital Athens, and some had a printed note of “Islamist content” inside, written in English, a police official said, without elaborating.
The General Secretariat for Civil Protection said an irritant, used in the production of adhesives and printing ink, was likely deliberately added to the envelopes.
Police sources said the anti-terrorism unit of police was conducting an inquiry.
Seven people reported a respiratory reaction after coming into contact on Wednesday evening with an envelope addressed to the rector of the University of the Aegean on the island of Lesbos, authorities said.
Some letters were intercepted before reaching the intended recipients.