A passenger rose from his seat and shot two thieves to death as they tried to rob people aboard a bus on the outskirts of Mexico City, police said Tuesday. It was the second vigilante-style shooting in two weeks, with a total of six alleged thieves killed.

An official with the Mexico State police force confirmed the latest shooting happened near midnight Sunday. One of the thieves died aboard the bus and the other died shortly afterward, said the official, who was not authorized to be quoted by name.

Local media reported that a third person — a female passenger aboard the small, van-type bus — was also shot and killed during the incident, but the official could not immediately confirm that.

The official said the shooter fled and had not been detained. The shooter in the similar incident that killed four suspected thieves in late October also has not been caught.

Passengers have been increasingly arming themselves in reaction to a surge of robberies aboard commuter buses in Mexico City's suburbs, a crime wave that has proved a major embarrassment for the government of Mexico State, which surrounds the capital.

Despite increased police presence aboard buses and at bus stops, robberies have continued and vigilante acts have risen. In August, local media reported that an armed passenger in another part of Mexico State shot at four thieves on a bus, killing two of them.

Mexico City's combination of heavy traffic and crime has led to several instances of motorists filming open banditry on expressways. Videos posted on social media sites show thieves approaching cars stopped in traffic, leaning in drivers' windows and apparently robbing them in broad daylight. City officials have stationed police temporarily on the sides of some expressways.