More than 1,000 police raided housing projects outside Paris early Monday, detaining at least 20 people in a bid to find rioters who led an outburst of violence last year, police said. Police raided Villiers-le-Bel and three neighboring towns as part of the investigation into November riots, police said. Those detained ranged in age from 19 to 31. Violence erupted in the town's housing projects, populated largely by families of immigrant background, after two teenage boys were killed in a motorbike crash with a police car. Police and local officials said it was an accident but many residents were unconvinced. About 100 police officers surrounded one building in Villiers-le-Bel across from a library and preschool that were burned down by rioters. It was not immediately clear whether there were arrests in that building. Police also spread through projects in nearby Sarcelles, Gonesse and Arnouville. Last year's flare-up in Villiers-le-Bel had stoked fears of broader unrest similar to the three weeks of rioting that swept poor neighborhoods across the nation two years earlier. Many of those rioters were of Arab or African background, frustrated by entrenched discrimination and isolation. In Monday's raid, police said they were targeting about 40 people suspected of attacking officers during last year's violence. At least 130 police were injured in the rioting. Two gang leaders in particular were targeted for organizing the unrest, according to police officials who spoke on condition of anonymity because they were not authorized to speak to the media. "It was a quasi-military organization," one police officer said of the riots. At the height of three nights of unrest, rioters fired shotguns at police. Prosecutors in the nearby city of Pontoise opened a judicial inquiry into attempted homicide against the assailants. Investigators issued an appeal for witnesses and promised monetary rewards for information leading to the shooters. Earlier this month, President Nicolas Sarkozy announced a sweeping plan to better integrate poor suburban youth and tackle the racism they often face in the job market.