The study focused on seven places - including Prince William and Frederick - that have adopted the federal 287 (g) program, which is in use by at least 68 state and local law enforcement agencies.
Under the program, local police officers are trained to determine the immigration status of people they arrest and flag those found to be in the country illegally to Immigration and Customs Enforcement. Once illegal immigrants are transferred to ICE custody, the U.S. government decides whether to begin deportation proceedings.
Although ICE has said it prioritizes deportation of people with serious criminal backgrounds, some of the jurisdictions participating in the program seek to remove as many undocumented immigrants as possible, regardless of their criminal background, the study found.
Frederick was among them, the institute found. In fiscal 2010, the county detained 198 undocumented immigrants through the 287 (g) program, and 120 were traffic offenders, the study says.
Prince William took a more target approach, the study found. The county detained 846 illegal immigrants in 2010, including 237 traffic offenders. In Loudoun County, 12 of the 47 people detained had committed a traffic offense.
In Frederick and Prince William counties, fewer than 10 percent of those detained were arrested for the most serious, Level 1 crimes, which include murder, rape and armed robbery. In Las Vegas, by contrast, more than 50 percent of illegal immigrants detained were arrested for the most serious crimes.
"The wide variation reflected in our study suggests that state and local actors, rather than ICE, are significantly defining the program's enforcement priorities," says the report, which based its findings on ICE data and interviews conducted in the seven localities.
Frederick County Sheriff Chuck Jenkins denied that the county is trying to deport as many illegal immigrants as possible but said that undocumented traffic violators are potential threats.
"How do you say that a [serious] Level 1 offender is more dangerous than an immigrant who's out driving a car illegally who could potentially hit someone and kill them?" Jenkins said, noting that those arrested are "in the country illegally to start with, and beyond that, they've committed a crime. How many free passes do you give someone?"
The report says that targeting serious criminals makes more sense as an enforcement strategy, particularly when government resources are limited.
Pursuing unauthorized immigrants who haven't committed serious crimes "takes away resources to go after what everybody believes are the high-priority criminals," said Marc Rosenblum, a senior policy analyst at MPI and one of the report's authors. "You can't have them be a top priority and then have the waitress or the gardener who's never broken a law except to be here illegally, you can't have them both be the top priority. When these state and local enforcement agencies fill up the detention centers and jails with low-priority cases . . . it compromises ICE's ability to do high-priority enforcement."
It also sows fear and distrust between local immigrant communities and the police, the report says.
Immigration advocates have long decried the 287 (g) program, saying that it leads to racial profiling and destroys relationships between police and immigrant communities.
Because of the controversy surrounding it, the program's growth has stalled. Another enforcement program, Secure Communities, has largely replaced it at the local level, although Secure Communities has been attacked for some of the same reasons.
Nationally, 686 jurisdictions in 33 states participate in Secure Communities, including the District and parts of Northern Virginia and Maryland. The program has been criticized in the Arlington County and the District, where local officials worry that it is discouraging undocumented immigrants from reporting crimes.
ICE has said it plans to expand the program to every state by 2013.
Corey A. Stewart (R-At Large), chairman of the Prince William Board of Supervisors and a supporter of both enforcement programs, said he thinks that allowing jurisdictions to apply the 287 (g) program differently is a positive thing, especially given variances in concentrations of immigrants in different areas.
"I would say that's a plus," he said. "I think it's an example of localities and states being the great laboratories for policy development."