(CNN) -- Immigration and Customs Enforcement agents have arrested nearly 400 illegal immigrants -- including 347 with prior criminal convictions -- over a three-day operation throughout the Midwest, the agency announced Friday.
The arrests in Illinois, Indiana, Wisconsin, Kansas, Missouri, Michigan, Ohio, Iowa, Minnesota and Nebraska were part of the agency's "Cross Check" operations that began in 2009 and are being held across the country.
"We are doing more than ever to prevent people from coming here illegally and we're doing more than ever to focus on those criminal aliens who are threatening our public safety," said Gail Montenegro, a spokeswoman for ICE's Chicago, Illinois, office.
Of the 370 people arrested this week, more than half of them had prior convictions for serious violent crimes such as armed robbery, drug trafficking and aggravated assault.
"These are crimes that have been committed here in the United States," Montenegro told CNN Radio. "They have criminal convictions for criminal conduct that they have committed here in the United States.
"We used a number of different leads," she said. "We tracked individuals with information we had available in our databases. We worked with other law enforcement and used information they had as well to develop our target lists."
Some of those arrested were affiliated with gangs or convicted of sexual crimes against minors. More than 50 were immigration fugitives who had outstanding deportation orders, the agency said.
The operation, which ended Thursday, was the largest of its kind for the agency, according to ICE.
"ICE is focused on arresting convicted criminals who prey upon our communities, and tracking down fugitives who scoff at our nation's immigration laws," ICE Director John Morton said in a statement. "The results of this operation demonstrate ICE's commitment to those principles."
CNN Radio's Shelby Lin Erdman contributed to this report.