Lawyer Benoît Côté, 51, and notary Marie-Josée Sills, 30, died in hospital after being shot at work on Thursday afternoon.
Côté was at one time the lawyer for Michel Dubuc, a man found dead in his home in Boucherville, Que., on Friday morning. The bodies of Dubuc's two sons, 21-year-old Jérémie and 19-year-old Gabriel, were also found in the home.
In 2013, Dubuc filed a $1.2-million lawsuit against Côté in relation to a failed business venture. The most recent court date in the case was last Monday.
On Friday, Quebec provincial police confirmed that the incidents in Terrebonne and Boucherville were related.
Police said Sills was likely just in the wrong place at the wrong time.