The army is scrambling to find how 13 .50-caliber and eight 7.62-caliber machine guns went missing on October 10 from its arsenal in the southeastern town of Barueri, 30 kilometers (25 miles) outside Sao Paulo.

Officials have voiced concern the weapons could wind up in the hands of organized crime, in a country where violent clashes between the security forces and heavily armed drug gangs are common.

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, which has opened an in-house investigation, initially ordered all 480 troops who work at the base to remain there for questioning, holding them for a week.

The army's southeastern command said Wednesday in a statement it had reduced the alert level at the base, "which means a reduction in the number of troops garrisoned."

However, around 160 soldiers are still on orders not to leave the base, it told AFP in an email.

The army said the stolen machine guns were "unserviceable" and had been taken to the arsenal to be repaired or decommissioned.

However, Sao Paulo state security secretary Guilherme Derrite said Saturday the theft could have "catastrophic consequences" if the weapons went to criminal gangs.