The Notorious B.I.G.’s 1997 murder remains unsolved. One of the Los Angeles Police Department sergeant detectives who worked the hardest to change that, Russell Poole, has died. According to The Los Angeles Times, the former detective died Wednesday (August 19), while meeting with detectives at the L.A. Sheriff’s Department. Although retired, Russell was apparently working on a cold case when he collapsed. Per reports, the cause of death is believed to be a heart attack.

Detective Poole’s investigation aimed to identify who fatally shot Christopher Wallace (a/k/a The Notorious B.I.G.) on the opening hours of March 9, 1997. Much of his theories and information would appear in Randall Sullivan’s book, LAbyrinth: A Detective Investigates the Murders of Tupac Shakur and Notorious B.I.G., the Implication of Death Row Records’ Suge Knight, and the Origins of the Los Angeles Police Scandal. In addition to its breakdown of what is often called The Rampart Scandal (a basis of the film, Training Day), the Atlantic-published text largely pointed to co-founder and CEO of Death Row Records, Marion “Suge” Knight as the man responsible for Biggie Smalls’ murder. Although incarcerated the night of the murder (for a parole violation stemming from the night Biggie’s then-rival Tupac Shakur was murdered six months prior) Poole’s research and largely anonymous sources alleged that Knight orchestrated the drive-by shooting upon the Rap star as an attack on his personal and business rivals. The shooter, in a mid-1990s Chevrolet Impala SS was alleged to be a man named Amir Muhammad (a/k/a Harry Billups). The theory also involved extensive cooperation from the Los Angeles Police Department in the murder (especially a Compton native and former label employee named David Mack), and examined Death Row Records’ hiring of off-duty cops from the department throughout the 1990s.

Three years after leaving the LAPD Homicide Unit, Russell Poole appeared in the 2002 Nick Broomfield-directed documentary Biggie & Tupac. He was featured alongside Biggie’s mother, Violetta Wallace, who has stated publicly that she aligns with Poole’s beliefs in regards to why her son was murdered. The theory’s accused, Knight, also briefly appears in the film, along with Death Row staffers, and LAPD officers attached to the controversial label. Here is an excerpt featuring Russell Poole:

In recent months, the retired Poole resurfaced in media. He proposed a theory to VICE regarding the drive-by shooting and subsequent murder of Tupac Shakur, which happened in Las Vegas, Nevada. This theory involved Suge Knight’s then-wife Sharitha, Death Row’s then Head of Security Reggie Wright, Jr., and another assailant in the Hip-Hop community, a former Priority Records artist and Snoop Dogg associate named Lil 1/2 Dead, who—as Poole alleges—was angry at Shakur for stealing a song “Brenda’s Got A Baby.” Notably, Reggie Wright, Sr., a Compton police officer, appears in Biggie & Tupac. This latest theory has never been substantiated.

The detective assigned to The Notorious B.I.G. murder after Poole, Detective Greg Kading, published his own investigation, Murder Rap, which disagreed greatly with Poole’s theories surrounding both celebrity murders—namely, the men who pulled the trigger, and in the case of Shakur—who orchestrated the attack.

In his career, spanning 1981-1999 before becoming an author and private investigator, Russell Poole also worked investigating the 1997 high-profile murder of Bill Cosby’s son, Ennis.

It is unclear if the cold case Poole was meeting with officials at the time of his death surrounded Christopher Wallace.

Ambrosia For Heads extends our condolences to the family of Detective Russell Poole.

Related: Former LAPD Detective Proposes A New Theory Surrounding Tupac’s Murder