Former Malaysian leader Mahathir Mohamad is under investigation for defaming the attorney-general in blog posts criticizing his failure to bring corruption charges against Prime Minister Najib Razak, police said on Monday.

The feud between Malaysia's past and current leaders has gripped the Southeast Asian nation for almost a year, with Mahathir repeatedly calling on Najib, his former protege, to step down over a financial scandal at indebted state-fund 1Malaysia Development Berhad.

In a Feb 5 posting on his personal blog, Mahathir wrote attorney-general Mohamad Apandi Ali had "no credibility", after he cleared Najib of any criminal offenses or corruption arising from a graft probe into a $681 million deposit in Najib's personal bank account.