MORONI (Reuters) - Authorities in Comoros questioned former president Ahmed Abdallah Mohamed Sambi on Tuesday over his role in a scheme to sell citizenship that investigators say was riddled with corrupt practice.

Sambi had been summoned to report to the gendarmerie’s investigations brigade on Tuesday morning but was instead questioned at his home, according to two sources and the summons seen by Reuters.

The prosecutor’s office issued an order preventing Sambi, another ex-president Ikililou Dhoinine, who succeeded Sambi, and 10 other senior officials from their administrations from traveling as they would be needed for questioning.

Comoros launched a program with the United Arab Emirates and Kuwait in 2008 to sell citizenship to stateless people in those countries, known as Bidoon, in return for cash to help develop the poor Indian Ocean archipelago.

However, an investigation by the Comoros parliament released earlier this year found that thousands of passports were sold outside official channels via “mafia” networks and at least $100 million of revenues went missing.

Sambi denied accusations made in the parliamentary report that he had personally profited from the citizenship scheme.

“It is a blatant lie … I challenge anyone to provide proof of the payments,” he told Al Watan, the main newspaper in Comoros, on Tuesday.

Anyone whose name had been put on a presidential decree authorizing citizenship had been checked by the police first, he said.

“I cannot comment on whether some people were able to find and exploit some loopholes as I was not aware,” he said.

A Reuters investigation published last year also found that two Iranians accused by the U.S. government of breaking sanctions were among those who bought Comoros passports via the networks that sold documents outside the official program.

President Azali Assoumani has said the scheme has since been suspended and promised to hold those who broke the law or embezzled money to account.

Sambi, who was in power from 2006-2011, has previously denied all accusations against him, saying they were cooked up by people trying to discredit him.