Investigating magistrates in the northern city of Milan have opened a probe into the threats against Silvia Romano and police have stepped up patrols around her home.
Romano was 23 and working as a volunteer in an orphanage in Chakama village in southeast Kenya when she was seized by gunmen in November 2018.
Prime Minister Giuseppe Conte met with Romano in Rome on Sunday. She said that she had converted to Islam during her detention of her own free will.
"People like her would be executed by US states that apply the death penalty," said one anonymous social media post.
"Have you ever heard of a Jew who, after being freed from a concentration camp, converted to Nazism and went home in an SS uniform?" wrote Simone Angelosante, a regional councillor for the far-right Islamophobic League party, referring to Romano's conversion.
"Am I happy about Silvia Romano being freed? Absolutely not. Now we will have one more Muslim and four million euros less," wrote Massimo Giorgetti, vice president of the northern Veneto region, according to the AGI news agency.
Giorgetti subsequently deleted his Facebook post.
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Italian media have said that the government paid a ransom to free Romano. Five people including three children were shot and wounded during the attack on the orphanage.
The government has denied paying a ransom, but media reports say that between 1.5 and four million euros were paid.
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