This study used a new approach to examine the causal effect of police on crime. Previous research has looked at this effect based on the prediction that crime will decrease when police presence increases without taking into account the positive bias resulting from an increase in police presence. The researchers selected a specific incident, a terrorist bombing at the main Jewish Center in Buenos Aires in 1994. One week after the attack that killed 85 and wounded over 300, the federal government assigned police to every Jewish and Muslim building in the country. Because the geographical distribution of these institutions could be presumed to be exogenous in a crime regression, this event constituted a natural experiment whereby the effect of police presence on crime could be determined. Data on car thefts per block were collected for the 9-month period surrounding the attack, from April 1 to December 31, 1994, in three neighborhoods in Buenos Aires. Information was also collected on the location of each Jewish institution in these neighborhoods. The researchers then estimated the effect of police presence on car theft in these neighborhoods. The results showed that blocks that received police protection experienced significantly fewer car thefts than the rest of the neighborhoods. Relative to the control group, car thefts fell by 75 percent in the blocks in which the protected institutions were situated. Thus the effect of increased police presence was large and extremely local. The results showed no evidence that police presence in a given block reduced car thefts one or two blocks away from the protected building. References, appendix, tables, figures