Anti-Corruption Agency (ACA) officers are required to attend refresher courses and programmes that reinforce the importance of their remaining 'clean' in carrying out their duties, the Dewan Negara was told today.

Minister in the Prime Minister's Department Mohd Nazri Abdul Aziz also said the officers are strictly vetted before they are absorbed into the agency.

He was replying Senator Ismail Kassim who asked about the steps taken by the government to ensure that the ACA is clean and efficient.

"They are officers who are entrusted to fight graft, so we expect them to have a stronger will than other officers," said Nazri.

He said three ACA officers have been charged to date for alleged abuse of power.

Senator Mohd Puad Zarkashi wanted to know if the government would set up a special commission to investigate high-ranking ACA officers accused of graft.

Citing the case of former ACA Director-General Zulkipli Mat Noor, said to have accumulated wealth beyond his means, Nazri said the police conducted the investigations.

"Of course, the ACA cannot investigate its own director-general, he was investigated by the police as announced by Inspector-General of Police Musa Hassan in a media statement. The investigation must not only look transparent, but also be visible to others."

He said Zulkipli's contract, which expired in March, was not extended because the tenure had been extended more than once, and not because the allegation.

"I don't want to make any statements that may show that I believe he is not guilty. Police have carried out their investigation and they will let us know the outcome soon," he added.

Zulkipli has denied the accusations levelled by a former senior ACA officer Mohamad Ramli Abdul Manan in a report sent to the prime minister and top cop before his retirement last December.