The cause of death was unclear and not stated in the NBC News report. Bulger was 89 and serving a sentence of life in prison, and had recently been transferred to the high-security Hazelton penitentiary in West Virginia, NBC reported.
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Henry Brennan, a defence lawyer for Bulger, said in an email he could not confirm or deny the report.
Officials at the Federal Bureau of Prisons did not immediately respond to requests for comment.
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