A lengthy grudge was at the heart of the deadly mass shooting at Brown University and subsequent murder of an MIT nuclear physics professor, law enforcement sources told ABC News.

According to the sources, the alleged killer, Claudio Valente, recorded a video in his native Portuguese explaining his motive for opening fire on a study group at Brown’s engineering and physics building on Dec. 13, killing two students and injuring nine others, before fatally shooting MIT professor Nuno Loureiro, at his home in Brookline, Massachusetts.

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