"Based on what we know now, people purchased what they thought was cocaine when it turned out to be a mixture of cocaine and heroin," Houston Police Capt. Richard Holland said.
Holland said three of the men in custody have admitted to delivering drugs to 16-year-old Jennifer Rivera, who died Sunday.
Jose "Kilo" Colunga, 19; Roman Gabriel "Horse" Juarez, 23; and Lucas Martinez, 23 were charged with conspiracy to distribute cocaine. They are scheduled to appear in court Monday.
A 25-year-old suspect, whose name was not released, was charged in state court with possession with the intent to deliver.
Harris County Sheriff's Maj. Juan Jorge said the fourth suspect told investigators he delivered drugs to three of the overdose victims and to an unknown number of other drug users who did not die.
All four could be charged with more serious state and federal counts. Holland said that decision will be made after the medical examiner's report is complete.
Medical Examiner Joye Carter said two more deaths this week are under investigation. None had been reported since the weekend, Holland said.
Holland said authorities are continuing to look for drug dealers who might have sold the mixture, though whether it is still being sold is unknown.
John Gartland, special agent-in-charge of the Drug Enforcement Administration's area office, said the deaths should serve as a grim warning about street drugs.
"There is no quality control to this stuff," he said. "When you buy it, you don't know what you're getting. Don't do it: People will sell anything for a price."
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