Jakarta. The national antigraft agency has barred Samin Tan, the owner of listed coal mining company Borneo Lumbung Energi, from traveling abroad for six months after naming him as a witness in a bribery investigation involving a coal-fired power plant in Riau.

Several lawmakers and a Singapore-listed company have been implicated in the case, which also saw one of President Joko "Jokowi" Widodo's ministers being forced to resign.

"The ban was imposed on the witness Samin Tan for the next six months to assist with the investigation," Corruption Eradication Commission (KPK) spokesman Febri Diansyah said on Monday (17/09).

The KPK has named three suspects in the case. They are Golkar Party secretary general Idrus Marham; Golkar lawmaker Eni Maulani Saragih, who is a former deputy chairwoman of House of Representatives Commission VII; and Johannes Budisutrisno Kotjo, a shareholder in Singapore-listed miner Blackgold Natural Resources.

Idrus resigned as social affairs minister last month after the start of the investigation.

Corruption investigators questioned Samin last Thursday on the money that was allegedly paid over to Idrus.

Johannes allegedly promised Idrus $1.5 million to ensure that state utility company Perusahaan Listrik Negara (PLN) signs a power-purchase agreement with the Riau-1 power plant.

Idrus allegedly either had knowledge of, or received some of the Rp 6.25 billion ($420,000) in bribes Johannes reportedly paid to Eni in March and June.

Eni had some of the money in her possession when investigators arrested her at Idrus's house in the Widya Chandra ministerial complex in South Jakarta in July, while she was attending a birthday party for the latter's child.

KPK investigators simultaneously also raided Johannes's office in Graha BIP in South Jakarta, just a kilometer from Idrus's home.

The KPK said it has completed its investigation against Johannes and is ready to bring him to trial soon in the Jakarta Corruption Court.