Persistent public suspicions about corruption and mismanagement that swirl around Egypt’s secretive deal to sell natural gas to Israel prompted Egypt’s public prosecutor on Friday to extend the questioning of former President Hosni Mubarak for 15 days, judicial officials said.

The announcement, carried by the government-run Middle East News Agency, came after the agency reported that the former oil minister, Samih Fahmy, and five other top officials had been imprisoned pending an investigation of the deal.

Adel el-Saeed, the prosecutor’s spokesman, issued a statement saying that among other issues, Mr. Mubarak was being questioned about gas exports to Israel at a low price that amounted to “hurting the country’s interests.” Egypt lost more than $714 million in the deal, the prosecutor said in a statement quoted by wire services a day earlier.

Selling gas to Israel was deeply unpopular in Egypt from the time the pipeline opened in 2008, given the sour public mood toward its neighbor, and it was a rallying point for the Tahrir Square protest movement since its start in January. But the deal was protected at the highest levels as long as Mr. Mubarak was in power. The key shareholder in the private company that brokered the deal was the president’s longtime friend, Hussein K. Salem, and a senior Egyptian official said the intelligence service also owned a slice of the deal.