A study of an Arizona-style immigration policy in Prince William County, Va., has found that it reduced the number of illegal immigrants in the county, but that its effect on violent crime was inconclusive. The study was conducted by the University of Virginia and the Police Executive Research Forum, a nonprofit group focused on improving police tactics, at the request of the county. It looked at data from 2007, when the policy was proposed, through 2009. Prince William County began enforcing the tough immigration law, similar to one that was passed later in Arizona and is now facing legal challenges, in 2008. The county's law required police officers to check the immigration status of anyone they had probable cause to believe was in the country illegally. The county executive, Corey Stewart, pushed the policy in a campaign that polarized residents. Hispanic groups criticized the policy as inflammatory. The county's police department, which paid for the study, expressed concern that the law would be expensive to carry out and that it would lead to accusations of racial profiling, and eight weeks later, it was suspended. It was later revised to apply only to those who had been arrested. According to the Migration Policy Institute, the county's foreign-born population more than doubled in the past decade, a rise largely attributable to the housing boom in northern Virginia. However, the report found that the number of illegal immigrants in the county dropped between 3,000 and 6,000 from 2006 to 2009. 'We are convinced that it's a clear result of the policy,' said Thomas M. Guterbock, a professor of sociology and one of the authors of the study. It is not entirely clear whether reducing the illegal population was one of the policy's objectives. Stewart said in an interview last week that it was a desirable goal, but that the earlier, stricter policy had not been workable. 'I believe that if someone is here illegally, they should be deported,' he said. 'But from a more practical perspective, we should be focusing on those illegal immigrants who are committing crimes.' Illegal immigrants represent just 6 percent of the perpetrators of all serious crimes in the county, a small enough slice that measuring the effects of the policy on crime has been tricky. Christopher Koper, director of research at the research forum and one of the authors of the study, said a significant finding was the sharp drop in aggravated assaults immediately after the announcement of the policy in 2007. But the drop might have been a fall-off in frightened immigrants reporting crimes, he said. 'We have no indication that the enforcement of the policy led to a reduction in crime,' Koper said. 'Crime trends have been steady.' Eric Byler, a documentary film maker whose film, '9500 Liberty,' captured the county's struggle with the law, argued that the policy, which was in place only in March and April of 2008, was too short-lived to have had much of an impact. The controversy the policy created had perhaps the most serious effect, he said. 'If anything this is the measure of the controversy's impact, not a measure of the policy's impact,' he said. 