RadarOnline.com broke the news that the Twilight beauty had been staying with Agnelli since fleeing from the home she shared with Pattinson.
"Rob knows Agnelli and has been calling him a lot since Kristen has been there," an insider tells us exclusively.
"He wants to know why Kristen is staying there at his house? He is super angry now and has accused Giovanni of sleeping with Kristen too."
Angelli produced 2010's Welcome to the Rileys which starred Kristen and James Gandolfini.
"Giovanni got a call from Pattinson around four this morning, threatening him if he has been sleeping with Kristen," the source says.
"Rob is beside himself over Kristen's betrayal. No one has ever seen him like this."