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BRIGHTON, Colo. (KDVR) — District Attorney Brian Mason announced Wednesday that after an “intense” investigation into five fentanyl deaths in Commerce City in February, there is not enough evidence to file charges.

Mason described the mass fentanyl-related deaths as “one of the largest mass incidents of this sort in the United States.”

Mason told FOX31 back in February that finding the source of the fentanyl was a “long shot.” Six months after the incident, Mason said in a statement Wednesday that “the evidence that exists today has not and will not lead us to an arrest or the filing of a charged case. We will continue to investigate any new leads that may come in.”

In a news conference, Mason described the case as “cold,” but maintained the investigation is open and they will continue to chase down any leads. Anyone with information that could help investigators can leave an anonymous tip with Metro Denver Crime Stoppers at 720-913-7867.

The 17th Judicial District Attorney’s Office worked with the Commerce City Police Department, the United States Drug Enforcement Agency, the United States Attorney’s Office for the District of Colorado and the North Metro Drug Task Force to follow leads, review surveillance, interview witnesses and process DNA and cell phone data.

“My heart goes out to the families and friends of those who died in this tragic incident in February,” Mason said in a statement. “I want to thank all of the law enforcement agencies who spent hours investigating these deaths. Addressing the fentanyl crisis is a collaborative effort and continues to be a top priority for all of us.”

Mason spoke about the investigation Wednesday afternoon. You can watch the full update on FOX31 NOW.