Prince’s heirs are suing the pharmacy store chain Walgreens and the Illinois hospital that treated him after he suffered a drug overdose just a week before he died.

Prince was 57 when he was found in an elevator at his Paisley Park estate on April 21, 2016. He died from an accidental overdose of the powerful synthetic opioid fentanyl.

About a week earlier, his plane made an emergency stop in Moline, Illinois, when he suffered from an opioid overdose and had to be revived by paramedics.

The wrongful-death lawsuit filed on Friday in Cook County, Illinois, alleges that a doctor and pharmacist at Trinity Medical Centre failed to appropriately treat and investigate Prince’s overdose, contributing to his death.

Walgreens declined comment due to the pending litigation. A message left with the hospital wasn’t returned.

Last week, no criminal charges were filed into Prince’s death after a two-year inquiry.

Prosecutors in Minnesota announced they found no evidence of intentional wrongdoing in the the singer’s death, reporting a self-administered overdose of painkillers as the cause.

“We simply do not have sufficient evidence to charge anyone with a crime related to Prince’s death,” Carver County Attorney Mark Metz told a news conference last Friday.

Prosecutors said they found counterfeit pills containing fentanyl in his home, but were unable to determine the source.

“There is no reliable evidence showing how Prince obtained the counterfeit Vicodin laced with fentanyl or who else may had a role in delivering the counterfeit Vicodin to Prince,” Mr Metz said.

Investigators found evidence that Prince suffered from severe pain for a number of years and that hundreds of various sorts of painkillers were found in his residence, according to Mr Metz.

The probe included searches of Prince’s computer, mobile phone records of his friends and interviews with associates.

Some of the pills were prescribed to his bodyguard, Mr Metz said, to protect the singer’s privacy.

But he said there was “no evidence that the pills that killed Prince were prescribed by a doctor”.

Mr Metz added that Prince may not have known he was taking a counterfeit Vicodin painkiller, which was found to be laced with potentially deadly amount of fentanyl.

“Nothing in the evidence suggests Prince knowingly ingested fentanyl,” Mr Metz said.

“In all likelihood Prince had no idea he was taking a counterfeit pill that could kill him.”

-with AAP