According to court documents and statements made in court, officials made eight controlled purchases of heroin, fentanyl and/or crack cocaine from Castro.
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During the sales, officials say Castro was assisted by 43-year-old Todd Peters of Hartford.
Castro has been detained since his arrest on Aug. 14. He is scheduled to be sentenced on Feb. 28, 2018.
Peters pleaded guilty to the same charge on Nov. 16 and awaits sentencing.