Attorney-general Apandi Ali said Kevin Morais could not have worked on prosecuting a "high-ranking government official" because the murdered deputy public prosecutor never worked on the 1MDB probe.
"What was claimed by the brother of the late Kevin is preposterous. Kevin was never involved in the 1MDB investigations," he told The Malaysian Insider .
"In fact, when he was transferred back to the AG's Chambers in early July 2014, there was no report in respect of 1MDB yet."
Besides 1MDB, investigators are also probing claims that RM2.6 billion and RM42 million were found in Prime Minister Najib Abdul Razak's bank accounts.
The RM42 million is allegedly from a former 1MDB subsidiary SRC International, but the firm says all its funds are accounted for.
Najib denies any wrongdoing.
Yesterday, Kevin's brother Charles Morais in a statutory declaration said he believes Kevin had initialed an alleged charge sheet against Najib, leaked by whistleblower website Sarawak Report .
Charles claims that Kevin told him he was working on a case involving powerful individuals .
Apandi earlier denied the existence of the charge sheet.
Sarawak Report said an anonymous whistleblower claimed the charge sheet was the reason for Apandi's predecessor, Gani Patail's termination.
The government says it is because Gani is sick and undergoing dialysis.
Charles said he believes Kevin was the anonymous whistleblower.
Kevin's body was found in a cemented oil drum, several days after he disappeared.
His body was only this week claimed by his brother Richard Morais, despite objections from Charles who insisted on a second post-mortem.