The 21-year-old represented Parma during his youth career, but the Ghanaian left the Italian outfit following their bankruptcy in 2015.
Nyantakyi is believed to have suffered from depression, and the former footballer has admitted to murdering his 43-year-old mother and 11-year-old sister earlier this week, with his elder brother Raymond Nyantakyi discovering the brutal scene on Tuesday night.
"I remember Solomon, he trained with us at Parma and I had him on the bench more than once during Serie A games," former Parma boss Roberto Donadoni told reporters.
"I remember he was very calm, even taciturn. If it really was him who did this, I can't think what went through his mind. He was a midfielder in the Parma youth academy, then I don't know what path he took after the bankruptcy."
According to various reports in Italy, Nyantakyi, whose father Fred is believed to be based in London, was stopped by police in Milan on Wednesday night, and quickly confessed to the crime.
Nyantakyi played in Serie D with Imolese last season, but his contract was not renewed this summer.