Det. Orlando Martinez said that when he interviewed Murray with two attorneys present, the doctor made up a story to protect himself.
"He was not being honest and forthright," Martinez testified.
Martinez said that when he began investigating Jackson's death in 2009, he at first concluded that the cause of death was accidental or natural.
But he said that when he discovered Murray's deep financial straits, he shifted his thinking and pondered whether he had discovered a motive for the pop star's death -- "financial gain."
Martinez was the second witness called Tuesday in the lawsuit that Jackson's mother and three children filed against Anschutz Entertainment Group, accusing the entertainment giant of negligently hiring and supervising Murray.
AEG says it was Jackson who employed Murray, and that the $150,000 a month the company was supposed to pay the doctor was an advance to the singer, much like the money it had loaned him to pay for his Holmby Hills mansion and production costs for the upcoming "This Is It" concert series in London.
The doctor owed hundreds of thousands of dollars in tax liens, child support and other debts, and he had closed his office to work with one patient -- Jackson, tying his financial future to him, Martinez said.
Martinez's testimony was part of the strategy by lawyers for the Jackson family to show that Murray was more concerned with getting himself out of his financial hole than caring for his patient, and that AEG should have checked him out.
Murray was convicted of involuntary manslaughter in 2011 for giving Jackson a fatal dose of the anesthetic propofol to help him sleep. He is completing a jail sentence.