Malaysian Anti-Corruption Commission (MACC) chief commissioner Abu Kassim Mohamed has vowed to get to the bottom of Prime Minister Najib Abdul Razak’s case involving billions of ringgit found in his private bank account.

In an interview with Oriental Daily , Abu Kassim revealed that pursuing the case had been a great challenge as this was the first time an investigation was being carried out on a serving prime minister.

"Nobody can influence the course of our investigation - this was what I had promised to the (MACC) officers at a meeting," he pledged.

The people will ultimately come to know the truth regarding the RM2.6 billion “political funding” that was deposited into Najib's personal bank account, he said.

"This is a long process and a challenging one; it involves many parties,” he said in a written reply to questions from the Chinese daily.

“We have summoned 90 (witnesses) from SRC International Sdn Bhd alone, while the RM2.6 billion case involves many (individuals) from overseas."

MACC has been probing SRC International, a former subsidiary of 1Malaysia Development Berhad (1MDB), over the allegation that RM42 million of its money ended up in Najib's bank account late last year and early this year.

He urged all parties not to speculate on the matter and promised that MACC would conduct the probe in a “free, transparent, and professional” manner.

Abu Kassim also acknowledged that the MACC investigation had been affected by criticism and speculation on the issue.

"We will submit the investigation report to the Attorney-General's Chambers once the probe has concluded. It is up to the Attorney-General's Chambers to decide on prosecution," he said.

The two-month medical leave Abu Kassim took at the height of the 1MDB, SRC, and RM2.6 billion donation scandal had sparked fierce speculation surrounding the anti-graft agency.

According to Abu Kassim, he was forced to take leave to undergo spine surgery after his back pain became excruciating.

While he was away, a number of key MACC investigators were arrested and their homes and offices searched as part of a police probe on the alleged leak of banking information relating to the Najib case.

There was also a government attempt to transfer two MACC senior officers but this was later aborted, following a public outcry.

This was followed by cabinet reshuffle on July 28, where then vocal deputy prime minister Muhyiddin Yassin and senior minister Mohd Shafie Apdal were axed. At the same time, Abdul Gani’s tenure as attorney-general was terminated.

The Public Accounts Committee's (PAC) probe into troubled government sovereign fund 1MDB was subsequently put off for three months.

He will bow to no one

However, on the high-profile investigation involving the premier, Abu Kassim stressed that he would bow to no one.

"I will not resign or seek early retirement due to pressures and factors other than my health," he said.

"Soon after Hari Raya (in July), the special task force met. I told (the then) attorney-general Abdul Gani Patail and Bank Negara governor Zeti Akhtar Aziz that I had to undergo surgery in August because of my deteriorating situation.

"I tried to postpone the surgery but my doctor disagreed," he revealed, adding he was forced to take painkillers to enable him to continue working.

He eventually returned to work in mid-October, two months after the operation.

Abu Kassim, whose tenure will end in 2018, hopes that he could lead the agency until December 2020.

He frowned at the attempted transfer of MACC special operations director Bahri Mohd Zin and strategic communication director Rohaizad Yaakob which he claimed had affected the morale of the anti-corruption agency.

The transfer, which was done while he was on medical leave, should never have happened in the first place, he lamented.

Abu Kassim said he had told his officers to be fearless in carrying out their duties.

According to him, more and more people no longer see MACC as government-controlled body involved in selective prosecution.