In 1978 his wife, Cecilia Fontana, was assassinated by poison intended for himself, sent by an unknown person or group. Other politically prominent families were also - unsuccessfully - targeted. This tragic incident has been widely blamed on political violence associated with the period of civilian-military rule in Uruguay, which began in 1973, after which Mario Héber and many other politicians were proscribed from political activities, but which he continued to pursue clandestinely.