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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. Play video content

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

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