Prince had been struggling from health problems for years, including a heart murmur, a friend of the legendary musician who died Thursday, said, according to Radar Online. Several celebrities expressed their grief over the surprise death, the cause of which has not been revealed yet.

Matt Fink — the keyboard player in Prince’s backing band for most of the 1980s, Revolution — told Radar Online: “I don’t know what’s happened, but the only thing I could think of is that he was more ill with something than he was letting on.”

Fink also told Radar Online that he did not believe Prince’s representative who gave flu as the reason for his death, referring to the late musician's plane making an emergency landing last week while he was returning from a concert in Atlanta. Fink said: “I can’t believe it was flu related. At that time, I speculated it was not good,” adding: “But I was glad he was well. I assumed he was fine, that he was healing.”

Addressing reports that suggested Prince was also suffering from epilepsy, Fink said, according to Radar Online: “He never discussed it. He brought that up much later after the band,” adding: “He told me he had a heart murmur as well.” However, Fink said they never discussed it.

A heart murmur is a reference to the sound produced when blood flows across one of the heart valves loudly enough to be heard through a stethoscope. It could indicate a narrowing or leaking of valves, or the presence of abnormal passages through which the blood flows in or near the heart.

Fink also said the band was in shock due to the sudden passing. “We’re in major shock right now,” he said, according to Radar Online, adding: “It’s very difficult to express how we (the members of the band) feel. It’s a tough time.”

A report by TMZ said that the “When Doves Cry” singer was given a “save shot” — usually given to cure avoid the effects of an opiate — after his plane made the emergency landing in Moline, Illinois last Friday. Once in the hospital, Prince was reportedly told to stay there for at least 24 hours but three hours later, when his team could not get him a private room, the singer left and went back to his home in Minnesota.

The report by TMZ also said that authorities investigating the cause of death are trying to get the medical records from the Moline hospital. Prince’s team has not yet commented on the rumors suggesting the singer died due to a drug overdose.

Prince was found unresponsive by the police in an elevator at his Paisley Park studio in Chanhassen, Minnesota. Emergency personnel tried to revive him using CPR but it did not help. He was pronounced dead at 10:07 a.m. CDT (11:07 a.m. EDT).