'American Idol' Doug Kiker's Appearance Prompted Pedestrian to Call 911

Doug Kiker was rushed to the hospital after a concerned passerby walked by him on the street ... and, the star's look was so worrying this person called paramedics.

Law enforcement sources tell TMZ ... an individual walking around in Denver passed Kiker on the street, took one look at him, and immediately called 911.

We're told this person told the 911 operator they were worried about his appearance, saying he may have been experiencing a drug overdose.

Emergency medical services units transported Kiker to the hospital ... where he died five days later.

Sources close to the situation -- not our law enforcement sources -- told us drugs caused Doug to have a heart attack ... landing him in the hospital.

Worth noting ... the medical examiner has not announced an official cause of death -- and, the investigation is ongoing.

As we told you ... Kiker's ex-fiancée, Valerie Cook, told us Kiker died Monday -- she said she hated that her kids lost their father. Doug's mom and brother also confirmed his death to us.

"The Singing Garbage Man" rose to fame on Season 18 of "American Idol" ... when he brought Katy Perry to tears with his rendition of Rascal Flatts’ “Bless the Broken Road." He was eliminated in the following round after singing Marvin Gaye's "Ain't No Mountain High Enough."

Kiker was 32.