Invoking the Boston Marathon bombings, a national conservative group has filed a lawsuit aimed at a Cook County ordinance that requires jail personnel to disregard federal immigration detainers.

Washington-based Judicial Watch says the county has no legal right to ignore the detainers, which are U.S. Immigration and Customs Enforcement requests that local jails hold specified individuals up to two business days beyond what their criminal cases require.

President Obama’s administration says the detainers, which help ICE put the inmates into deportation proceedings, are crucial for focusing immigration enforcement on criminals.

Cook County officials say detainers also erode community trust in local police. In 2011, the County Board approved an ordinance that halted detainer compliance by the county’s massive jail. ICE abruptly lost convenient access to hundreds of immigration violators each year. Lawmakers in other parts of the country, meanwhile, approved bills modeled after the policy.

The suit, which claims federal law preempts the ordinance, asks Cook County Circuit Court to strike down the local measure and compel Sheriff Tom Dart to comply with the detainers. The suit accuses Dart of “failure to carry out his legal duties under both federal and state law.”

At a Monday press conference Judicial Watch President Tom Fitton pointed to last week’s news events. “In light of the Boston Marathon bombings, there is a national-security component to these detainers by ICE.”

Authorities say two Chechen immigrants — one a permanent-resident visa holder, the other a naturalized U.S. citizen — are suspected of having planted the bombs that exploded April 15 in Boston.

Judicial Watch is representing the suit’s plaintiff, Chicago resident Brian McCann, who is the brother of a pedestrian killed in a 2011 hit-and-run collision in Chicago’s Logan Square neighborhood. The alleged driver, a Mexican immigrant named Saúl Chávez, had a DUI conviction. He was arrested and charged with the hit and run. A Cook County judge set the bond at $250,000.

ICE suspected Chávez was in the country illegally and slapped a detainer on him. But after the county enacted the ordinance, Chávez posted $25,000 — the required 10 percent of the bond. He walked free and went missing.

“Dart is thumbing his nose at the federal government and replacing federal immigration priorities with Cook County’s own immigration policy,” Fitton said. “Releasing these criminal aliens before they can be taken into custody by ICE endangers the public.”

Fitton echoed Homeland Security Secretary Janet Napolitano and ICE Director John Morton, who have said the Cook County ordinance threatens public safety.

That claim was the subject of a WBEZ investigation that found that inmates freed as a result of the ordinance had not reoffended or jumped bail more than other former inmates had.

Dart’s office, in a statement late Monday, pointed to the sheriff’s support for allowing the county to honor ICE detainers for inmates charged with violent offenses and inmates with a number of prior convictions.

Chip Mitchell is WBEZ’s West Side bureau reporter. Follow him on Twitter @ChipMitchell1 and @WBEZoutloud, and connect with him through Facebook and LinkedIn.