Prince's heirs have filed a wrongful death suit against the drugstore chain Walgreens and an Illinois hospital where the singer was treated, then released, the week before his fatal overdose in 2016.

Minnesota Public Radio's Matt Sepic reports that attorneys representing Prince's estate allege that Trinity Medical Center, in Moline, Ill., where Prince's plane made an emergency landing on April 15, 2016, failed to appropriately diagnose and treat his overdose.

The singer was given two doses of Naloxone, a drug designed to reverse the effects of an opioid overdose.

Prince died on April 21 at age 57. He was found unresponsive at his home in the Minneapolis suburb of Chanhassen. The suit alleges his death was a direct result of the inadequate care he received in Illinois the previous week.

On Thursday, Carver County Attorney Mark Metz said that the two-year investigation into Prince's death concluded that he died after taking a counterfeit Vicodin pill laced with the powerful and dangerous opioid fentanyl. However, Metz said, investigators were unable to find out how Prince obtained the pills, so there was no evidence to charge anyone.

The lawsuit, filed in Chicago's Cook County, also names Walgreens Co., alleging that pharmacists at two of its Minnesota branches dispensed "prescription medications not valid for a legitimate medical purpose."

Both Walgreens and the hospital parent company declined to comment.

As The New York Times reports, "Based on documents related to the criminal investigation released on Friday, prosecutors believe that Prince had likely overdosed [on April 15] on what he believed to be prescription opioids like Vicodin, but were actually black market versions containing the much more powerful synthetic opioid fentanyl. Authorities determined that, without knowing, Prince most likely took a counterfeit drug containing fentanyl" that resulted in his fatal overdose six days later.

At the hospital, The Associated Press says, "Prince refused medical tests but was asked what drugs he took. Documents show a pill that he had with him, which was marked as Vicodin, was sent to the pharmacy for testing. A hospital pharmacist said it appeared to be Vicodin and returned it to Prince."

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