It’s been almost a year since the 4/21 opiod-related overdose death of Prince, but despite intense scrutiny, according to a New York Times probe many questions remain about the cause of the Rock Hall of Fame musician’s demise. And, given the series of dead ends investigators have encountered so far, it’s possible the true story may never be known.

The basic facts remain: the 57-year-old musician known for living a clean marijuana- and alcohol-avoiding, vegetarian lifestyle suffered from chronic hip pain for many years. But investigators still don’t know how he came into possession of the powerful opioid fentanyl, which has been the cause of a rash of overdoses and death over the past few years. We also know that six days before his death Prince’s chartered jet had to make an emergency landing after he overdosed on the way home from a concert and he had to be revived.

There have been no arrests to date of anyone who might have supplied Prince with fentanyl, despite sweeps of the usual suspects. The Carver County Sheriff’s Office is running point on the probe along with the Drug Enforcement Administration, with the findings eventually going on to the U.S. attorney’s office, which will determine if any prosecution is warranted.

In these kinds of joint investigations the agencies typically focus on doctors and pharmacies, according to the Times, to see if painkillers were “improperly or illegally distributed to an individual.” So far, the sheriff’s office has talked to people who were at Paisley Park that day and, as of February, the inquiry was still active. The bad news is that if Prince’s fentanyl came from the black market — which appears to be the case — it will be difficult, if not impossible to track down the source without a paper trail.

The two initial focuses of the probe were Dr. Michael Schulenberg — who treated Prince before his death and was at his home the day he died with some test results — and Dr. Howard Kornfeld, a California opioid addiction specialist. Schulenberg had prescribed undisclosed medications to Prince and Kornfeld’s son, Andrew, arrived at Paisley Park after Prince died with a small dose of Suboxone, an anti-addiction agent used to revive overdose victims; he was reportedly not legally authorized to administer that treatment.

Neither doctor, nor Andrew Kornfeld, appears to be the focus of the ongoing investigation according to the Times. Schulenberg — who left his job at the North Memorial Health Care in the days after Prince’s death — has not said what medication he prescribed for Prince and there’s no indication that investigators think it was an opiate. His lawyer told the paper that Schulenberg has “fully cooperated” with the investigation and has not been questioned further by police since his voluntary interview on 4/21/16.

Dr. Kornfeld continues to run a treatment center in California and his son is applying to medical schools, according to the paper.

“Prince’s death has raised the profile of the opioid crisis even further,” said Dr. Chris Johnson, chairman of the Minnesota Department of Human Services Opioid Prescribing Work Group. The singer’s demise came after fentanyl — which is as much as 50 times more powerful, and cheaper to produce than heroin and often disguised as more expensive prescription pain pills — had not been seen on the local black market in the Minneapolis area, but experienced a sudden surge around the time of his death.

In the meantime, the questions persist and the answers appear further away than before. To read the full Times story, click here.

This story originally appeared on Billboard.