The brother of “Law & Order: Criminal Intent” star Kathryn Erbe said today that he was unwitting Facebook friends with her alleged stalker.

Jonathan Erbe said that when he accepted the “friend” request from Charles Nagel, “I thought he was someone I knew or was associated with my business.”

He was neither. During opening arguments, prosecutors described him as a stalker whose fixation with the stunning “Law & Order” beauty became chilling and abusive.

Testifying at Day 1 of Nagel’s criminal trial in Brooklyn federal court, Jonathan Erbe said it quickly became apparent that Erbe wasn’t who he thought he was.

He said Nagel started sending him messages complaining about getting kicked off “Law & Order” set locations and asking him to pass messages to the mother of two. One of the messages was, “I’m your greatest fan in the world. We all know without fans, there are no shows.”

The California resident said the messages “made me feel very uncomfortable. I un-friended him.”

Prosecutors say Nagel – a Batman nut who named one of his daughters Batgirl – often blogged about his feelings for Erbe.

Among the things jurors were told he wrote about her were, “I want to look in her eyes and make love” and, “Obsession is only around the corner from love, but it is not a bad thing . . . Kathryn is everything and everything is Kathryn.”

Lawyer for Nagel — who pleaded guilty in 1996 to sexually accosting a young girl in Philadelphia — said he’s simply a “misunderstood fan.”