The LAPD

This theory ties in with Suge Knight. LAPD officer Russell Poole, who was a lead investigator on Biggie's murder, accused other LAPD officers of having connections to Death Row Records and Suge Knight, who he thought planned Biggie's murder. 

Poole believed that Knight had Biggie murdered as revenge for Tupac's death. He was ordered to stop his investigation on the case, and retired in 1999. 

Poole died of a heart attck in August 2015, while he was discussing the case with LA County Sheriff homicide detectives. At the time, he was wokring on a book about the murders.