Four people arrested for ties to an attempted coup in Niger in December have confessed to the plot and asked for clemency, the defence minister said on Wednesday.
At least 13 people, including a civilian, had been arrested, Defence Minister Mahamadou Karidio said.
Nigerian President Mahamadou Issoufou said on national television on Dec. 17 that the coup attempt had been foiled.
There have been several coups in the landlocked West African nation, which the West sees as an important ally against terrorism, since independence from former colonial ruler France in 1960. Issoufou took power a year after a coup in 2011.
He is expected to win a second term in a February election but critics have said he has become increasingly authoritarian.
Lawyers went on a 24-hour strike on Monday to protest against what they called arbitrary detentions by the secret service, which the bar association said was denying access to clients.
"I can confirm with honour that at least four people have confessed to having prepared a coup d'etat, starting with their leader," Karidio told journalists on Wednesday. "They asked for clemency."
Karidio did not name those said to have confessed but said a head of an artillery battalion, an air base commander and the army's former inspector-general were among those arrested.
Family members of the accused were not immediately available for comment.
The defence minister said the presumed head of the botched coup plot had a list naming 10 other officers, but it was not yet clear whether they were involved.
Another 10 civilians and three customs officers were under investigation for ties to the plot, Karidio said. Military sources said they were being held for questioning by the president's secret service.
Amadou, once a friend to the president, was arrested upon his return from exile in November on suspicion of ties to a baby-trafficking ring. On Jan. 9 he was authorised to stand in elections but was denied release from prison two days later.
Opposition candidate under investigation
Karidio added that Hama Amadou, a principal opposition figure and presidential candidate, had been questioned as part of the investigation.
Amadou declared himself a political prisoner, his lawyer said on Wednesday, meaning he is now likely to spend time leading up to the February polls in jail.
Amadou was one of 15 candidates approved by the constitutional court to stand in presidential elections. His lawyer, speaking on a private television station, said Amadou made the decision after the appeals court turned down his plea to be released.
"We will not seek an appeal. We will no longer ask for conditional release," said Souley Oumarou, the candidate's lawyer. "Our client considers this no longer to be a judicial affair, but a political affair."
The leaders of Amadou's Moden political party have also been jailed, Oumarou said.
About 30 other members of Niger's political and economic elite, including Amadou's wife, were charged with procuring newborn infants from neighbouring Nigeria to sell to wealthy families
He has publicly denied the charges, which he says are politically motivated. All of the other people arrested under the investigation have been granted conditional release.