Venezuela has arrested the state oil company’s boss for the western region and eight other executives at PDVSA, according to an internal company memo and a half-dozen sources in the OPEC member’s oil industry.

It was not immediately clear why Gustavo Malave and the other employees were apprehended, though a series of corruption probes are under way at PDVSA and have entangled other employees.

The sources said Malave was arrested on Sept. 4 in Zulia state, Venezuela’s traditional oil-producing region near Colombia, in what would be one of the highest-profile detentions of a PDVSA executive.

PDVSA, the prosecutor’s office and Malave did not immediately respond to a request for comment.

Separately, Venezuela’s new chief prosecutor Tarek Saab on Aug. 31 announced he was investigating “spectacular” overpricing in a dozen contracts in the nation's Orinoco oil belt, on the other side of the country.

The reputation of PDVSA—short for Petroleos de Venezuela SA—has been tarnished in recent years by graft investigations involving high-profile staff. The company has blamed the problems on a small group of employees and executives, and promised a war on corruption.

In 2016, the opposition-led congress said $11 billion was lost at PDVSA between 2004 and 2014, when Rafael Ramirez was in charge of the company. He denied the allegations.

The Caracas-based company is the financial motor of leftist President Nicolas Maduro’s government, but is reeling from low oil prices, mismanagement, and lack of investments.