Army troops and federal highway police set up checkpoints on Rio de Janeiro's major access roads Wednesday, trying to prevent thieves from fleeing with rifles stolen from a Brazilian army barracks last week.

Some 1,500 troops and police officers have occupied nine shantytowns searching for the guns stolen Friday by seven gunmen wearing army-issue camouflage gear and ninja masks. The gunmen overpowered three guards, stole the weapons from a small depot and sped away in at least two cars waiting outside the building, the army said.

On Wednesday, acting on a tip that the bandits were planning to take the guns out of Rio, troops deployed along the city's main highways and randomly checked passing vehicles, said Col. Fernando Lemos, spokesman for the Eastern Military Command.

On Tuesday night, drug gang members in the Mangueira shantytown fired tracer bullets in the air in an attempt to intimidate the troops, who did not return fire, Lemos said.

On Monday, a youth identified as Eduardo dos Santos, 16, was killed, apparently by a stray bullet, as troops exchanged gunfire with assailants in the downtown Providencia shantytown.

The army has not arrested anyone and has not found any of the weapons. Lemos said the troops could leave the slums if they do not recover anything.

The massive sweep to recover a handful of weapons was unusual. Some speculated it was linked to October's national elections, with the government trying to curb crime and win public support.

Rio is one of the world's most violent cities, with an annual homicide rate of nearly 50 per 100,000 people.

In 2003, the army was summoned to control a crime wave on the eve of the carnival holiday. Troops also have patrolled the streets during international summits, including a 2004 meeting of Latin American presidents in 2004.

At the Earth Summit in 1992, 15,000 troops were deployed to protect foreign visitors, and tanks were stationed with cannons pointed at major shantytowns.