The machine guns, some capable of bringing down aircraft, went missing on October 10 from a military arsenal in the southeastern town of Barueri, 30 kilometers (25 miles) outside Sao Paulo.

Rio de Janeiro police "found eight of the 21 machine guns... The weapons were inside a stolen car" in a favela in the western area of the city, Governor Claudio Castro said on social networks.

According to Rio police, traffickers bought the weapons as part of a war they are waging against law enforcement in that region of the city.

The four .50-caliber machine guns and four other 7.62-caliber machine guns were wrapped in black plastic bags in a parked vehicle, according to images released by the governor. No arrests were reported.

Authorities suspect there was "military participation" in the theft, discovered on October 10, but which may have occurred a month before, General Mauricio Vieira Gama, chief of the Military Command Staff of the Southeast, said at a press conference Thursday.

Some 160 soldiers remained on lockdown in Sao Paulo as investigations advanced, according to the local press.

The incident is the biggest theft of weapons from the Brazilian military on record, according to the anti-violence group Instituto Sou da Paz, which monitors misappropriation of firearms from the country's security forces.

Illegal in Brazil for non-military use, .50-caliber machine guns are more than a meter (three feet) long, weigh up to 58 kilograms (128 pounds) and are capable of taking down aircraft.

Designed for combat, 7.62-caliber machine guns weigh about 4.5 kilos and can fire up to 700 rounds per minute.

The army said the weapons were "inoperative" and had been moved to the arsenal for repair or dismantling.

Still, Sao Paulo authorities warned that the theft of the weapons could have "catastrophic consequences" if criminals deploy with them.