Matthew's Journal

The Harry Potter Lexicon has confirmed what I heavily suspected since I finished reading Harry Potter and the Half-Blood Prince. The author of the note Harry finds in the fake Horcrux, RAB, has been identified as Regulus Arcturus Black. Regulus Black is Sirius Black’s brother. According to Order of the Phoenix, Regulus was a Death Eater, but was killed when he tried to back out. Now we know that not only did Regulus try to back out, he tried to take a piece of Voldemort with him.

The Lexicon hasn't revealed their source on this information and Jo Rowling hasn't de facto confirmed this information either. However, I have very little doubt that the information is accurate.

See the direct link here: http://www.hp-lexicon.org/wizards/regulus.html