THREE YOUNG FRIENDS ALL WALK INTO A WASHINGTON, D.C. HOME AND ONLY TWO COME OUT ALIVE - AT LEAST ONE OF THEM IS A KILLER, BUT WHICH ONE?

"48 HOURS" INVESTIGATES IN "WHAT HAPPENED IN APT. 1601?"

Saturday, April 2, 2016 (10:00 PM, ET/PT)

Three friends, all with big futures ahead of them, headed to a tiny studio apartment near Washington D.C. after a night of barhopping. Sometime afterwards, one of them was butchered with a knife. The two survivors then told police they don't know what happened.

Richard Schlesinger and 48 HOURS investigate the 2013 death of Mark Waugh and the police search for the truth in "What Happened in Apt. 1601?" to be broadcast Saturday, April 2 (10:00 PM, ET/PT) on the CBS Television Network.

"It's like a horror movie," says Montgomery County police officer Dean Skiba, describing the bloody scene in Apt. 1601. "You could see it was up all over the wall, all over the ground."

Waugh, 23, was stabbed to death in the apartment where his friends, Rahul Gupta, a graduate student, and Gupta's girlfriend, Taylor Gould, a biomedical engineer, lived.

Gupta and Gould, bloody and drunk, according to police, were interrogated immediately afterward. Police thought they would quickly solve the case, especially after Gupta seemingly confessed. "I caught my buddy and my girl cheating," Gupta told police. "I killed my buddy."

But then, as the alcohol wore off, everything changed. Gupta insisted he had no idea what transpired. Taylor had a similar story. "I don't remember what happened," she told police. "I told you everything I did remember."

Police struggled to believe Gupta and Taylor completely forgot the events leading up to their friend ending up dead. "They could both remember up until a certain point, but the crucial 45 minutes nobody seems to remember anything," Detective Paula Hamill says.

Investigators then tried something highly unusual in interrogations that may have changed the face of the case. They put Gupta and Taylor together alone, and monitored their conversations. This conversation ultimately would decide who went home, and who would be charged with murder.

So how does Waugh's life end? And could anyone figure out who killed him? Schlesinger and 48 HOURS search for answers through interviews with police, investigators, friends and attorneys involved in the case. 48 HOURS: "What Happened in Apt. 1601?" is produced by Josh Yager and Dena Goldstein. Tom Costantino and Kevin Dean are the editors. Peter Schweitzer is the senior producer. Susan Zirinsky is the senior executive producer.

48 HOURS: "What Happened in Apt. 1601?" is the second part of a Saturday night double feature. The first 48 HOURS: "Gone," to broadcast at 9:00 PM, ET/PT, features Tracy Smith and 48 HOURS' investigation into the disappearance of Michelle Warner and the unusual path her on-again, off-again boyfriend Mark Castellano took to tell his side of the story on "The Dr. Phil Show."

Chat with members of the 48 HOURS team during the broadcast on Twitter and Facebook. Follow 48 HOURS on Instagram. Listen to 48HOURS podcasts at Play.it.