Sanctuary states have been highly successful in shielding criminal illegal aliens from arrest and deportation by Immigration and Customs Enforcement (ICE) agents, new data reveals.

California, Colorado, Connecticut, Illinois, Massachusetts, New Jersey, New Mexico, New York, Oregon, Vermont, and Washington are among the nation’s largest sanctuary jurisdictions as they impose statewide policies that prevent local law enforcement from cooperating with ICE.

Colorado Department of Corrections data published by KRDO 13 in an exclusive report reveals the extent to which the state’s sanctuary policy has ensured that ICE agents do not bother with issuing detainers for illegal alien inmates as officials are not allowed to honor such detainers.

The detainers ask officials to hold illegal aliens in their custody until ICE agents can take over custody, putting them in federal detention and beginning deportation proceedings.

Since 2009, KRDO 13 reports, ICE detainers placed on illegal alien inmates in Colorado Department of Corrections custody have been cut in half from nearly 1,300 in 2009 to fewer than 530 this month.

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The data comes as KRDO 13 revealed that three illegal aliens accused of murdering a 30-year-old man in El Paso County, Colorado had previously been arrested, some convicted, in the state but were released from custody rather than being turned over to ICE agents.

In June, Breitbart News exclusively reported how human smugglers take full advantage of sanctuary states to carry out their operations in smuggling illegal aliens throughout the United States.

Whistleblowers told Breitbart News at the time that California’s sanctuary state policy, for instance, is a vital tool for human smugglers. In cases where federal agents are trying to track down smugglers and the illegal aliens they are smuggling, whistleblowers said California local police almost never intervene to help with arrests.

“If the illegal aliens have seatbelts on, the cops won’t call us to report smuggling. So long as they’re following state laws,” a whistleblower said. “The sheriffs cannot, by law, pick up migrants even if they had an ICE detainer after they finish their sentence, they’ll just let them loose.”

John Binder is a reporter for Breitbart News. Email him at jbinder@breitbart.com. Follow him on Twitter here.