FBI and police to use luminol at night to search for blood in Olney house of horrors Philadelphia police and the FBI will use luminol at nightfall to search for human blood in the Horsch residence in Olney, as the investigation into as many as seven missing women continues. PHILADELPHIA, Pennsylvania -- Philadelphia police and the FBI will be back at the so-called house of horrors Monday night in Olney to continue the investigation into the Horsch residence at 417 W. Chew Ave. U.S. investigators have been coming and going from the Horsch home for months. They work in the daylight, searching for clues of foul play and human remains. On Monday night, they will wait until it is dark. Law enforcement will look for signs of human blood using luminol at nightfall.Luminol Search Targets Missing Women CaseThe search is part of the ongoing effort to uncover what happened to as many as seven missing women tied to the home and its deceased owner, Raymond R.C. Horsch. Luminol is a presumptive test for blood, according to retired FBI supervisor Richard Marx. Marx oversaw evidence recovery in some of the most complicated investigations, including Pulse nightclub and the World Trade Center.CBS News Philadelphia asked him about luminol, which is being used in the West Chew Avenue case. When sprayed on trace amounts of what is believed to be blood, it gives off a very faint luminescence, a blue light, Marx said. It has to be relatively dark in the room to see the light because it is a very faint chemical reaction.Police sources say they have located valuable evidence in their work to uncover what happened to the missing women. The investigation remains focused on the Horsch residence and its connection to the disappearances. Raymond R.C. Horsch, the dead homeowner, has been identified as a central figure in the case.Eugene Horsch Remains In CustodyHis son, Eugene Horsch, remains locked up on drug and weapons charges pending a federal trial. His attorney has declined to talk about the case or comment on whether Eugene has spoken with investigators. Time will tell if detectives are able to find evidence of foul play or traces of blood inside the Horsch residence. Marx said investigators should be able to recover evidence from long ago.Because items that often retain blood are very porous. They are almost like a sponge, Marx said. So, if you have carpet, if you have hardwood floors, they can seep down into the gaps of the floors. So spraying the chemical can often detect blood that is many, many years old.Sources say excavation equipment will be brought in Tuesday to dig up portions of the backyard of the home as the search for any possible human remains continues. The nighttime luminol search marks a shift in the investigation, which has so far been conducted during daylight hours. The use of luminol requires darkness to detect the faint blue glow that indicates the possible presence of blood.Forensic Experts Explain Blood DetectionMarx explained that the chemical reaction is extremely sensitive and can reveal trace amounts of blood that might not be visible to the naked eye. This makes it a valuable tool in cold cases and investigations where evidence may have been cleaned or degraded over time. The FBI and Philadelphia police have not commented on what specific evidence they hope to find during the nighttime search.However, the continued presence of investigators at the property suggests the case remains active and that authorities are pursuing all possible leads. As many as seven missing women have been linked to the home and to Raymond Horsch. The investigation has drawn widespread attention and raised questions about how the disappearances went unnoticed for so long. The search for human remains is expected to continue in the coming days, with excavation equipment arriving Tuesday.Excavation Equipment Arrives For Backyard DigAuthorities have not said how long the dig will last or what they expect to find. Neighbors and community members have expressed shock and concern as details of the case have emerged. The quiet Olney neighborhood has become the focus of a major law enforcement operation. Eugene Horsch remains in custody on unrelated drug and weapons charges.His federal trial is pending, and his attorney has declined to discuss the case or whether Eugene has cooperated with investigators. The use of luminol is just one part of a broader forensic effort. Investigators have been collecting evidence from the home and surrounding property for months, hoping to bring closure to the families of the missing women.Marx noted that porous surfaces like carpet and hardwood floors can retain blood for many years, making it possible to detect evidence even after a long period of time. This technique has been used successfully in other complex investigations. The FBI and local police are expected to continue their work at the property through the night and into Tuesday. Further updates are anticipated as the investigation progresses. By Joe Holden, Alicia Roberts and Tom Gardiner