KUALA LUMPUR (Reuters) - Malaysia will file additional charges against the wife of former Prime Minister Najib Razak, the anti-graft agency said on Wednesday, as authorities step up investigations into the former first couple.

Rosmah Mansor, wife of Malaysia's former Prime Minister Najib Razak, leaves a court in Kuala Lumpur, Malaysia October 4, 2018. REUTERS/Lai Seng Sin

Najib and his wife, Rosmah Mansor, who was for years criticized for her extravagant lifestyle, are at the center of a wide-ranging graft investigation launched after a stunning election victory and return to power as prime minister in May by veteran leader Mahathir Mohamad.

Najib has been charged with 38 counts of graft since his election defeat, mostly to do with state fund 1Malaysian Development Berhad (1MDB). Rosmah was last month charged with 17 money laundering offences.

Najib and Rosmah have pleaded not guilty to all charges.

Rosmah will face several charges on Thursday regarding solar power supply and installation projects in schools in the state of Sarawak, Malaysia’s Anti-Corruption Commission said in a statement.

Authorities did not say what the charges would be.

Rizal Mansor, a former special officer to Najib, will also be charged in connection with the same case.

The anti-graft agency said Tengku Adnan Tengku Mansor, a former minister in Najib’s cabinet, will also be charged on Thursday related to a land sale.

The 1MDB scandal was one of the major reasons for Najib’s election loss.

The fund is the subject of corruption and money-laundering investigations in at least six countries.

In civil lawsuits over the last two years, U.S. authorities allege that $4.5 billion was siphoned from 1MDB and about $700 million was diverted into Najib’s personal bank accounts.

U.S. prosecutors filed criminal charges against two former Goldman Sachs bankers this month as part of their 1MDB investigations.

Malaysian financier Low Taek Jho, described by U.S. and Malaysian authorities as central to the 1MDB fraud, was also charged. He remains at large.