BRUSSELS—Belgian police sent an internal note around in recent days warning that a terror suspect arrested three days after the Brussels attacks was carrying material that could be used to infect people with fatal illnesses like typhoid and cholera, a police official confirmed. But prosecutors separately dismissed the threat that the material posed.

The note was earlier reported by the website of Belgian newspaper Derniere Heure earlier this week. On Saturday, the website published the communication, which the police official...