The group are linked to IS and had plans to attack the Rio events
Ten people have been arrested in Brazil on suspicion of planning terrorist attacks during next month's Olympics in Rio de Janeiro.
Federal investigators have said they are searching for a further two suspects - and the group is accused of pledging allegiance to Islamic State.
"There was an order between them to start training in martial arts and to (obtain) ammunition and weapons with which they could carry out an act," justice minister Alexandre Moraes said.
One of the suspects had allegedly contacted an underground weapons site in Paraguay to buy an AK-47 assault rifle.
It is believed the group, who had used Whatsapp for their communications, did not know each other personally.
Mr Moraes said there were no specific locations being targeted by the suspects, adding: "It was an absolutely amateur cell, with no preparation at all - a disorganised cell."
The anti-terror probe had been carried out across nine Brazilian regions, but all of the arrests were made in the southern states of Sao Paulo and Parana.
According to the SITE Intelligence Group, a previously unknown group have been referring to themselves as "Ansar al Khilafah Brazil" - promoting IS propaganda in English, Portuguese and Arabic.
Officials in Brazil have said concerns surrounding terrorism at the upcoming Games have "reached a higher level" following last week's deadly truck attack in the French city of Nice.
Additional roadblocks and extra security cordons have been enforced, and body searches will be performed at visitors before they can enter Olympic venues.