Two officials from Reliance Industries and Reliance Power, which are owned by Mukesh Ambani and Anil Ambani respectively, were brought before a court in New Delhi on Saturday following their arrests on Friday evening.
Three employees working for other energy sector businesses also appeared in court, including the Essar conglomerate, which is controlled by the wealthy Ruia family, and Cairn India, the oil exploration division of billionaire Anil Agarwal's London-listed Vedanta Resources.
Read MoreHow big a deal is Modi's defeat in Delhi?
India's oil minister Dharmendra Pradhan promised a full investigation into the scandal, which emerged last week. This followed a police sting operation which led to the arrests of a series of middlemen on suspicion of illegally obtaining and then selling documents to the companies involved.
"Nobody is above [the] law. Nobody will be spared," Mr Pradhan told the Press Trust of India news agency this weekend. "This government will not permit anyone to breach the system or bend the rules."
Reshmi Khurana, head of India at security company Kroll, said the investigation was likely to attract extensive publicity given the high-profile businesses involved, all of whom have well-funded lobbying operations in New Delhi.
"These companies are names to reckon with," she said. "But this is a wider problem in the Indian system, when any sector with lots of regulation attracts layers of middlemen who you can deal with to get information, even if that means breaking rules."
More from The Financial Times:
Greece scrambles to send draft reforms to EU institutions
Shanghai exchange to open doors to foreign short sellers
Disunited Kingdom: UK — a new order?
Foreign companies operating in heavily regulated industries such as power, telecoms and defense often complain in private that domestic operators gain an unfair advantage through their ability to access information about forthcoming government decisions.
"We always assumed that our competitors would know anything going on in the ministry the minute it happened, and long before it became public. It was like magic," said the head of one global business operating in India, speaking on condition of anonymity.
Reliance Industries and Essar said they have launched internal investigations into the arrests of their employees. A spokesman for Reliance Power said it would "ensure full co-operation with the investigative agencies". Cairn India declined to comment.