Cops were looking into the possibility that Celeste Rivas was killed inside the Los Angeles home where singer D4vd was living … because TMZ has learned investigators who searched the home were looking for blood.

Law enforcement sources tell TMZ … LAPD’s Scientific Investigation Division took part in the 12-hour search of D4vd’s home, using luminol to look for trace amounts of blood.

The fact LAPD used luminol is significant … it’s a chemical used by forensic investigators to detect blood at crime scenes. Luminol reacts with the iron in hemoglobin, glowing blue when it comes into contact with blood in dark rooms. Think of it like a black light for finding blood.

As for why cops were looking for blood, it’s significant because Celeste’s body was found in a Tesla parked near the house. The Tesla is registered in his name.

We’re told investigators didn’t just look for traces of blood on floors and surfaces … they also looked in sink drains. It’s unclear if the luminol turned up any blood.

Our sources say cops also swept the home for fingerprints and collected hair samples. We do not know if the prints and samples are significant in the investigation.

As we reported … D4vd seemed to be living in the home, which his manager, Josh Marshall, was renting.

TMZ contacted LAPD’s Robbery Homicide Division last week after speaking with Celeste’s mom, who told us she had evidence the unidentified body was her daughter. We provided the lead detective with her name and number. Days later, the body was identified as Celeste.